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What is OBD?

A generic term simply used to describe On Board Diagnostics. OBD can refer to OBD1 or OBD2

OBD1 was the first diagnostic stage and OBD2 is the second stage now in modern cars (and all cars in the USA from 1996)


What is OBD and OBD2 and what are its benefits? 

OBD stands for “on-board diagnos­tics”

There are 2 types of OBD: They are known as OBD1 and OBD2

OBD1 is the early diagnostic system used in the USA prior to 1996 and in Europe prior to 2001 for petrol engine cars and prior to 2004 for diesel engine cars.

OBD2 is a computer-based system built into all 1996 and newer light-duty cars and trucks in the USA and afterwards adopted in Europe. 

Most petrol cars sold new in Europe from 2001 and most petrol cars sold new in Europe from 2004 have the OBD2 system. NOTE: Many commercial vans, trucks and 4x4 vehicles in Europe are not OBD2 standard until after 2007.

OBD monitors the performance of some of the engines’ major compo­nents, including individual emis­sion controls.

The system provides owners with an early warning of malfunctions by way of a dashboard “Check Engine” light (also known as a Malfunction Indicator Light or MIL, for short). By giving vehicle owners this early warning, OBD protects not only the environment but also consumers, identifying minor problems before they become major repair bills.

For example, by identifying a relatively inexpensive repair like the replacement of a faulty oxygen sensor, OBD can save the owner the cost of replacing the catalytic converter later.

By helping to ensure that the vehicle operates within its original design specifica­tions, OBD can help save con­sumers money by making sure fuel isn’t wasted as a result of, for example, a loose fuel cap or incom­plete combustion. OBD stores infor­mation about the malfunction detected and helps the repair garage to diagnose and fix the vehicle.


Where can I find the older OBD1 connector in a car?

Normally always under the bonnet (hood) in the engine compartment. The OBD1 diagnostic ports are not a standard shape and size and some will be round in shape and others will be oblong or square with various numbers of pins in them. The number of pins in an OBD1 port varies in car models


Where can I find the modern OBD2 connector in a car?

The OBD2 connector (port)  in cars from 2001 must normally be situated within 18 inches of the steering wheel in the vehicle. Usually they are positioned under the dashboard, low down directly under the steering wheel or in the center console, or under a plastic trim cover next to the fuse box inside the car. All OBD2 ports are 16pin although not all the pins are used and the port is a standard size and shape. ( D shape)  


What is an OBD protocol?

An OBD protocol is the commuincation language used in the car for the transmission of diagnostic data. A diagnostic tool must be compatible with the protocol used by the car to be able to link to the car's diagnostic system via the OBD port. If the diagnostic tool is incompatible with the diagnostic system in the car, the user will receive a "link error" message because the diagnostic tool is unable to link to diagnostic system


There are currently 5 OBD2 protocols as follows: 

J1850 PWM (pulse width modulation)  used by Ford Motor Company and Mazda

J1850 VPW (variable width modulation) used by General Motors and in light trucks        

ISO9141-2 = older protocol in Chrysler, European, and Asian vehicles between 2000-2004 

ISO14230-4 KWP2000 (keyword protocol 2000) commonly used in cars from 2003

ISO 15765-4 CAN-BUS = first introduced in 2004 then mandatory in all vehicles from 2008


There are also 4 different variations of CAN-BUS detailed below.

Each with different length and Bus speed 

(1)   ISO 15765-4 CAN (11 bit ID,500 Kbaud)

(2)   ISO 15765-4 CAN (29 bit ID,500 Kbaud)

(3)   ISO 15765-4 CAN (11 bit ID,250 Kbaud)

(4)   ISO 15765-4 CAN (29 bit ID,250 Kbaud)


What is EOBD?

It is the diagnostic standard for cars made in Europe based on the original OBD2 USA standard from 1996. EOBD is used in most petrol cars sold new in Europe from 2001 and most diesel cars sold new in Europe from from 2004.


What is JOBD?

Japanese On Board Diagnostics for cars made and sold in Japan


Is it true that asian import cars in Europe often have OBD1 diagnostics?

Yes. Many asian import cars, Hyundai, Kia etc ,petrol and diesel, are not OBD2 standard until after 2004


What is OBD11?

It's the official term for OBD2 


What is a Generic Fault Code?

A generic fault code is a code that means the same in all cars. Example: PO500 means speed sensor. The code PO500 means speed sensor in every car and not just the car you are testing. There are a large number of generic fault codes in the OBD2 system but each of the different codes has the same meaning in all cars


What is a manufacturers fault code? 

Manufacturers trouble codes are unique to a vehicle of a specific year, make, model (and possibly engine size). They are assigned by the vehicle manufacturers to cover situations not already covered by the generic fault code database. Manufacturers codes normally begin with P1 

Manufacturers codes are known as "specific" or "enhanced" codes. They are not generic fault codes, they are extra fault codes for the benefit of car dealerships which enable them to access more information from the car. There are hundreds of these extra codes covering many faults in a wide range of different systems and components in the car.

The diagnostic data read by dealership technicians is also read through the OBD2 port. These extra codes show things like knock sensor operation, FI pulse width, ignition voltage, individual cylinder misfires, transmission shift points, ABS brake condition and much more. There may be over 300 readings available in a car  depending on each manufacturer and each car model.

All vehicles vary in the readings they will support because some cars have more electronically monitored devices in them than others. Diagnostic tools vary widely in the number of these manufacturers codes they can read.  Basic diagnostic tools will show only the generic OBD2 codes. The more expensive diagnostc tools can show the full range of generic OBD2 codes and usually all of these extra manufacturers enhanced codes  


What are the most common fault codes? 

The most common are Powertrain codes known as P codes 

There are also Body codes known as B codes

And then there are Chassis codes known as C codes






Generic P codes all start with P0 and these are the P Code categories below:

P0001 - P0099......Fuel and Air Metering & Auxiliary Emission Controls

P0100 - P0199..... Fuel and Air Metering

P0200 - P0299......Fuel and Air Metering (Injector Circuit) 

P0300 - P0399......Ignition System or Misfire 

P0400 - P0499......Auxiliary Emissions Controls 

P0500 - P0599......Vehicle Speed Controls and Idle Control System 

P0600 - P0699......Computer Output Circuit 

P0700 - P0899......Transmission 


B codes all begin with B0 and cover side airbags, seatbelt tension, seatbelt loads, seat recline and car seat sensors etc 

C codes all begin with C0 and cover things outside the car such as brakes, suspension, steering, tyres, exhaust 


What is the MIL in a car?

Malfunction Indicator Light. Often referred to as an Engine Management Light or Engine Warning Light or Check Engine Light .The light is automatically switched on when the diagnostic system in a car detects a problem and stores a fault code.

Use a diagnostic tool to read the fault code/s. Make a careful note of the code/s and then simply use the tool to clear the codes. The OBD system is very sensitive and in many cases you will not actually have to fix anything after you clear the fault code/s unless the engine warning light turns on again. If after clearing the fault code/s several times, the fault persists, it must be fully resolved. 


I can't install diagnostic software because my antivirus says it contains a virus. What should I do?

This is a very common problem when customers mistakenly think diagnostic software they have contains harmful viruses or trojans If you are confident that the software has come from a reliable source always turn off antivirus before you try to install any software. Software usually has .exe, and other essential files to run programmes. Antivirus works on a beter to be safe than sorry basis and prevents these files installing in your computer in case they are trojans or viruses. Antivirus will usually compare the names of files on the software disc  with the names of rogue files in it's own database. If there are any file names that match or are sometimes just similar, the AV will block those files from your computer so you can't use the software and it will flag it up as virus or trojan.   


What is a diagnostic Interface?

A diagnostic interface is a piece of electronic hardware which fits between the car and a laptop computer. A cable from one end of the interface is plugged into a laptop and a cable on the other end of the interface is plugged into the diagnostic port in a vehicle to veiw diagnostic data directly on the laptop screen. The interface and some software which is installed in the laptop enables the user to carry out diagnostics in the car and data can be saved in the laptop


I am having trouble with my diagnostic interface as I can’t get it to connect to the car? 

There can be various reasons for this but the starting point is to check that the interface is compatible with the diagnostic system in the car. Determine if the car is OBD1 or OBD2 and check which OBD system the interface is designed to work with. 

Next, check the software and the interface have been correctly installed in your computer. You cannot use some sofware directly from the CD or DVD drive, sometimes you have to actually install it in your computer before you can use it so always read the instructions carefully.

The COM PORT number used by any interface must match the COM PORT number allocated for the interface in the computer

Check the instructions for the interface, they frequently don't work on COM PORT numbers higher than number 5 

The COM PORT number allocated in your computer for the interface is shown in "DEVICE  MANAGER" in your computer. You can look down the list in Device manager to see the COM PORT number there in the section named "COM and LPT". 

Whatever software you use must also match this same COM PORT number. The COM PORT number used by any software can be changed on the main page of the software. There will be a drop down box to make the change somewhere on the main page of the software you are using

As an example, the installation default settings for the Elm 327 interface cable are as follows:

Select a COM PORT number between 1 -5 in device manager and click ok or save
Bits per second:  9600
Data bits:  8
Parity:  None
Stop bits:  1

Note: The Elm 327 does not work on a COM PORT number higher than COM PORT number 5

- If you are using a USB to serial converter not all of them are compatible. Please ensure you check they are OK regarding operating system, RAM size etc.

- Some laptops have an IR port which may need to be disabled to avoid a conflict between ports

It is normally essential to turn off any antivirus before installing any new software or it will probably fail to work

After making changes to settings your computer may have to be restarted in order to recognize them

(If you are sure your computer is not at fault and the hardware and the software are installed properly but it still won't work:

Take your computer and the interface and try it in another car. If it still won't work in another compatible car, the software or the interface would appear to be faulty.)


What does it mean if the engine waning light turns on while I'm driving? 

If the light comes on and stays on, the OBD system has detected a problem. Your vehicle might have a condition that wastes fuel, shortens engine life, or causes excessive air pollution. If left unaddressed, these conditions could also damage your vehicle and lead to increasingly expensive repairs. For example, the diagnostic system can identify a loose or missing fuel cap (which wastes fuel and contributes to pollution) or an engine misfire which can lead to severe or permanent engine damage.


What should I do if the engine warning light goes out on it's own and stays off? 

Nothing. If the problem that caused the light to come on is addressed, the diagnostic computer will turn the light off. This is not an indication of a faulty diagnostic system. In fact, the system is doing its job by verifying that a problem temporarily existed but has since been corrected; perhaps a loose fuel cap was tightened or a fouled spark plug was cleared. Your vehicle needs no special attention unless the light comes on again.


Can I clear fault codes in an engine with the engine running?

No. The engine must not be running. The diagnostic system will not reset itself and the engine warning light will not go out while the diagnostic system in the car is running and monitoring the engine


Can I do any damage to the vehicle by clearing the fault codes in them?

None whatsoever. The primary purpose of all automobile diagnostic tools is to read and clear fault codes in vehicles


Can I disconnect the car battery to clear fault codes?

It's possible with most cars made before 1997 and some a few years after that but it's not a good idea. When all power is removed from the car all diagnostic information is lost but a genuine fault in the car will still be there when the battery is re-connected. Radio and alarm settings will be lost and the car may not restart. If the car is re-started it is likely to run very badly and may be difficult to drive for many miles until the car has re-learnt all it's engine settings.


I just cleared an engine fault code, why is the engine management light still on?

If you cleared the fault code the engine warning light will usually go out when you restart the engine unless there is an ongoing problem which will then have to be fixed to keep the light off


I keep clearing an engine fault code so why does the engine warning light show again a few days later?

It's a warning light to tell you the diagnostic system thinks something is wrong. When there is a real fault you must fix it to stop the light coming back on. When you clear a fault code it does not fix any fault, it just resets the diagnostic system by clearing the memory in the ECU. In the case of an ongoing problem which needs to be fixed, the car will quickly restore the fault code and trigger the engine warning light again. This will usually happen after the car has been driven at different speeds for al least 5 miles again when the engine has completed a "drive cycle"


The engine warning light in my car is constantly blinking whilst drving, what does that mean? 

The engine warning light gives three types of signals. Occasional flashes indicate brief malfunctions. It stays on if the problem is of a more serious nature, affecting the emissions output or safety of the car. A constantly flashing engine warning light can be a sign of a major problem eventually causing some serious damage if the engine is kept running.

A constantly blinking warning light indicates a severe engine problem such as a catalyst-damaging misfire. The exact problem must be identified and fixed as soon as possible to avoid an expensive repair bill. Switch the engine off as soon as possible when it's safe to stop. Do not to drive the car at high speed or with excess weight (such as towing or carrying heavy equipment)  


I bought a diagnostic tool to use with different cars but it doesn't link with some cars  - why not?

The tool must be compatible with the diagnostic system in each car. A tool for OBD2 will only link to OBD2 systems and a tool for OBD1 systems will only connect to OBD1 systems. Tools are available to link with both systems, however, they are more expensive than a tool which is only designed to link to just one of the systems. Tools for both OBD1 and OBD2 systems are professional tools. Make sure the diagnostic tool is compatible with the diagnostic system in the car. It's essential that the tool and the system match, otherwise it's impossible to get a link between the tool and the car


Can I use an engine fault code reader to reset airbags and ABS?

Definitely not if the tool is only an engine fault code reader. Engine fault code readers do not have functions for any systems outside the engine. Of course engine fault code readers are available with functions for a few extra systems oustside the engine, however, separate tools are normally required to link to each system in each car therefore you require a collection of different tools for different jobs in different cars. If  you had a small repair garage it would be sensible to invest in one or two of the more expensive professional diagnostic machines to cover most of the diagnostic systems in a wide range of different cars  


Can I do any damage to a vehicle by connecting a diagnostic tool?

None at all. The two longest pins in the connecting plug of any diagnostic tool are both earth pins. Because the earth pins protrude further than all the other pins in the plug, they safely make the first contact in the vehicle diagnostic port and protect the vehicle electronics in the event the car has a wiring or other serious fault which might result in damage to the tool or the system in the vehicle. In addition,the diagnostic port is protected by a fuse in the car fuse box which will immediatley blow to prevent any possible damage. The fuse is often shared with the cigarette lighter, the radio or both.. A diagnostic tool will not damage any car but an incorrectly wired car can damage a diagnostic tool and it may not be immediatley obvious the tool has been damaged since it may still connect to some cars but not in others  


I want to connect to a car with an OBD1 system but the tool I have is only for OBD2 systems. Can I just use an OBD1 adapter cable with the OBD2 tool?

No. Using an adapter cable doesn't change the system in the car. You have to match the actual tool with the system in the car. OBD2 tools can only link to OBD2 systems. However, most of the more expensive professional tools are designed to connect to both OBD1 and OBD2 systems, you can use various adapter cables to connect to OBD1 systems with that sort of tool


Can I use a diagnostic fault tool to reset my service indicator? 

You can't using a basic tool because service intervals are not part of the diagnostic system in any car. The service indicator is not a fault, it is a reminder.However, professional tools will usually have a service reset function or alternatively an inexpensive service reset tool can be otained and used for that sole purpose.

No tools are required to reset service in many very modern cars since it can be done just using the controls around the steering wheel. The proceedure varies in different models. Details may be found in a car owners handbook or on the internet


What is live data?

You already know if the car is running badly but live data should show you the cause when you connect a diagnostic tool and run the engine. Live data will show various sensor values and other information such as firing etc with the engine running which you can then easilly compare with the manufacturers default settings for the car. By comparing the data from the car with the manufacturers default settings, you can quickly identify the problem in the car


What is Freeze Frame?

It's a "snapshot" of the car system taken at the exact time the car developed a fault. This snapshot is stored in the car and shows exactly what the car was doing immediatley before the engine warning light was triggered. You can then compare the information with the manufacturers default settings in the car to identify the cause of the problem. Even basic fault code readers will usually have a function to read the freeze frame. Sometimes if the car completes three drive cycles without a re-occurrence of the problem, freeze frame data will be erased.


I was sure I cleared an airbag light using a diagnostic tool but the airbag light is still on - why?

This usually means the ecu in the car has not responded to the command sent by the tool to clear the light or the light went out for a few seconds and came on again because there is an airbag problem that needs to be fixed before the light will stay off permanently. A possible cause could also be a wiring fault or bad connection (under one of the seats) in the airbag system or maybe underneath the steering wheel or in a door panel or possibly the airbag control module is faulty. The fault must be found and fixed before the light stays off permanently.


Can I use a diagnostic tool to find out why my car won't start? The engine warning light is not on

It's unlikely that the car has stored a fault code if the light is not on. The reason for the non-starting, possibly something mechanical, is not something included in the diagnostic monitoring system in the car so no cause has been detected and stored as a fault code. In these circumstances a diagnostic tool will not help until the engine is running again and then a tool with live data function can be used to check settings. 

Think about all the circumstances immediatley prior to the problem, what was different? You will need to do some old fashioned detective work by a process of manual checking and elimination


How do I know if a car is OBD1 or OBD2?

When a car is OBD2 there will normally be a label with "OBD11" on it under the car bonnet (hood) near the top of the radiator

Please read the diagnostic info section on this website for extra information about OBD1 & OBD2 


What is the best source of information about fault codes?

The best source of all fault code information is Google. Simply type "fault code" followed by the code into the Google search box to get lot's of detailed information posted by people who have discussed their own fault code problems with others on the net. You can read what the fault codes mean and how other people fixed the faults in their cars, often in great detail. Youtube is another good source where you can find a wide range of videos showing how to fix faults


Is a hand-held code reader better for me than a laptop diagnostic interface cable?

It depends what you want to do with a diagnostic tool. Hand-held code readers are low priced, small and robust, and they are very quickly and easilly plugged into a car for instant fault code reading. Because they are highly portable they are easy to carry around and keep in the car. Whilst modern hand-held diagnostic tools cover all OBD2 protocols they are usually limited to just engine fault diagnosis in cars with OBD2 systems so most code readers are only suitable for simple engine fault code reading. Code readers normally only have very basic diagnostic capability, they are primarily for the DIY market

However, there are quite a few hand-held code readers covering extra diagnostic systems outside the engine such as ABS, Airbags etc, especially for OBD1 and OBD2 systems in Volkswagen/Audi/Seat and Skoda (VAG) cars 

There are other hand-held scan tools covering all OBD2 engine systems plus OBD1 engine, ABS, AIRBAG systems etc in certain manufacturers cars. Autel make a number of these tools: JP701, EU702,US703 and FR704. These tools cover Japanese, German & European, American and French car manufacturers 

On the other hand, the possibilities when using an interface cable to connect to the car with diagnostic software via a laptop computer are potentially endless. Many diagnostic functions may be available depending on the software used. Profesional interfaces and software are not cheap but it is possible to obtain cables and software which provide dealer level functions to all systems in a car which would be impossible using a code reader 


Are all diagnostic tools updateable?

Most hand-held OBD2 code readers are not updateable because their OBD2 (engine) code reading function never change so there are no updates produced for them even though "lifetime updates" may be promised. Software used with interface cables can usually be upgraded to provide more diagnostic functions as later versions of the software are produced. More expensive professional diagnostic tools can always be updated to provide increased functionality. Updates for the professional diagnostic tools can be expensive but frequent software updates are not normally necessary since control units in cars do not change often from one year to another 


Can I use a code reader to change settings in a car?

No. A code reader is just a reader. You can see but not change. A basic code reader will of course enable you to read and clear engine fault codes. When you erase the codes the engine warning light will go out. You need a professional tool to do coding and to make system changes and you need an even more professional tool to do prgramming 


Why can't diagnostic tools fix faults instead of just showing fault codes and clearing warning lights

Tools do not fix faults. People fix faults using tools.

Good tools are essential to make it possible and much easier to fix problems. The better diagnostic tools don't just read and clear fault codes. They can show various live systems data and an array of detailed and vital information for analysis and fault diagnosis but they are still not magic wands. A competent person in possesion of all the diagnostic fault information ultimately has to fix the fault   


Can the OBD system be deactivated, or modified?

When it comes to emissions, any modification that changes the vehicle from a certified configuration to a non-certified configuration is considered tamper­ing: this applies to both vehicle owners and repair facilities and is therefore an offence. Replacing a catalyst with a straight pipe is one traditional example of tampering. Likewise, overriding the OBD system through the use of high-tech defeat devices, non-certified computer chips, etc., would also be considered tampering. If the system is illegally modified it must be repaired back to its original certi­fied configuration with certified “performance chips” or appropriate aftermarket parts


For those in the U.S.A please see the links below to OBD FAQ's :