Click on a picture for a bigger view

Figure 1
10-pin ALDL Test Plug connections.

Figure 2
Location of the 10-pin ALDL plug in a Nova engine bay.

Figure 3
Close up of the 10-pin ALDL plug in a Nova engine bay.

Figure 4
ECU warning lamp on the dash board.

Figure 5
Location of 10-pin ALDL Test Plug in a Calibra.

Figure 6
Location of 10-pin ALDL Test Plug in an Astra-F.

Figure 7
16-pin ALDL Test Plug connections.

Figure 8
Location of 16-pin ALDL Test Plug in Astra MK4 within Fuse Box.

What are 'fault codes'?

Fault codes are recognised problems or faulty sensors to the EMU (Engine Management Unit) which is a ECU (Electronic Control Unit). The ECU relies on the sensors on your car to make it run at it's optimum performance at all times. When a sensors goes wrong or there is a fault with the engine, the ECU sees this problem and logs the fault code within it's memory (NOTE: not ALL problems are logged, depends on what system your car has). Often the ECU warning light on the dash board may light up, indicating to you that there is a problem.

NOTE: The ECU warning lamp usually needs to be on for more than 30 seconds before the ECU logs a fault code. Otherwise it could consider it an intermittent fault and ignore it.

Do all ECU's handle fault codes?

If your engine is fuel injected then it needs a ECU to control the injection timing. All Vauxhall models that have injection need a ECU to do this and hence they will have the facility to log fault codes. If your engine is run on a carburetor, then it has no ECU, hence no fault codes can be logged or read.

How do I read the fault codes?

There are two common ways to do this. The first is to use a TECH1 or a TECH2 reader which is an expensive bit of equipment. Some dealerships could charge you £50 just to connect it up to your car. The second option is FREE and YOU can do it....with a paper clip.

The paper clip method flashes the ECU warning lamp on the dash, and you read the amount of flashes and pauses. The chart supplied in this article tells you what the fault code means. The paper clip is used to short out two pins in the ALDL connector (also know as 'diagnostics plug' or 'test connector'). These are commonly coloured blue and can be located in the car or within the cabin, depending on model of car and fueling system.

It doesn't matter if the engine is running or not when the flash codes are being read out. In fact it is beneficial to have the engine running since on some ECU's, if it doesn't see the engine running, it will produce a fault code straight away and gives a misleading fault code.

Please note on newer ECU cars in the region of 2000 onwards, most now use ECU codes which can not be read out via the paper clip method. You will have to consult a ECU/fault code specialist or your nearest Vauxhall/Opel Dealer.

What pins do I short out?

If you have Motronic or GM-Multec fuel system, then you'll find a 10-pin ALDL connector. To read the flash codes, then short pins A and B (see figure 1).

If its a Simtec 56.0 or 56.1 fuel system (i.e. Ecotec engine) then you'll find a 10-pin ALDL connector. To read the flash codes you need to short pins A and B (see figure 1).

If its a Simtec 56.5 fuel system (i.e. Ecotec engine) then you'll find a 16-pin ALDL connector. To read the flash codes (P codes) you need to short out pins 5 and 6 (see figure 6).

Can I damage anything if I short the wrong pins?

Yes it is possible but unlikely, depending on what pin you short out. Most of them are 0v connections, but there are live +12v supply pins on there as well depending on which ALDL plug you have. If you are in doubt, don't do it! Double check the diagrams shown.

Where do I find the ALDL connector on the car?

Again if you have a Motronic or GM-Multec system, then these are commonly found in the engine bay or within the fuse box. Look for a 10-pin connector which is usually plugged into a blanking socket (i.e. not actually connected to anything). See figures 2, 3 and 5.

However if you have a Simtec 56.0 or 56.1 system, then the 10-pin ALDL connector can also be found in the engine bay or within the fuse box. See figures 2, 3 and 5.

However if you have a Simtec 56.5 system, then the 16-pin ALDL connector can be found in the fuse box area or under the trim to the hand brake or under the bulge in the carpet beside the passenger front seat. See figures 7 and 8.

Just before you start, make sure you note the following:

• If you don't have one, buy a Haynes manual for your car and have this is front of you if you find a fault and wish to investigate it.
• You'll need a paper clip and a pencil and paper to jot down the fault codes as they are flashed up.
• You do not need to start the engine but it doesn't hurt. Just switching on the electric's is enough (i.e. dash lights come on).
• If the paper clip is inserted in the wrong 2 pins, it is unlikely to damage anything, but double check you have the correct connections in the first place.
• For the ECU to log a fault, the ECU warning lamp on the dash usually has to be on for more than 30 seconds.

OK, got all the above? Then follow the steps below (remember to click on the pics for a bigger view):

The Motronic or GM-Multec fuel systems use a 2 digit flash code. The engine does not have to be running when reading the codes, although it doesn't hurt if it is.

Once the wire link is in place and the electric's are on, the ECU warning lamp will begin to flash out the codes (wait for the initial boot up check that lights up all the warning lamps). Each fault code is repeated 3 times and then it moves onto the next one (if any). Once at the end of the logged fault code list, it will go to the beginning again, giving you plenty of time to note down the code (on each fault code shown, there will be a pause before it shows the next one). It will carry on like this forever until you remove the key from the ignition or the battery runs flat. To clear the fault codes, simply remove the battery leads for a few minutes and then reconnect.

For example, lets say there was a ECU warning lamp noted and the owner wanted to read what fault code was logged. The owner has already put the wire link in place and switched on the ignition. A flash is indicated by an astrix (*) and a pause is shown as a dash (-).

*-** (1 flash, 1 pause and another 2 flashes = code 12)
*-** (12)
*-** (12)

**-*** (23)
**-*** (23)
**-*** (23)

*-** (12)
*-** (12)
*-** (12)

**-*** (23)
**-*** (23)
**-*** (23)

*-** (12)
*-** (12)
*-** (12)

Owner then takes the key out of the ignition.

Lets go through what has happened. After switching on the ignition, the dash board lights up showing the self check, after that all the lights go out and the ECU warning light begins to flash the error code. The first code is 12 (one flash, a pause and then two flashes). This is repeated 3 times.

Code 12 at this moment means initiation of diagnosis. After that came code 23, which means 'Knock control module', i.e. there is most likely a faulty unit or a bad connection at its plug. After code 23 has been repeated 3 times it flashes a separator code, which is code 12.

It flashes this 3 times before going to the next fault code. The next code is code 23 again which means its finished showing the logged fault codes and has started back at the beginning. The owner can switch off the ignition. A healthy car should display a stream of code 12's. To find out what fuel system you have, read the "Fault Code Chart" below.

The Simtec 56.0/56.1 fuel systems use a 2 digit flash code. The engine ideally needs to be running when reading the fault codes, otherwise it may show incorrect fault codes.

Once the wire link is in place and the engine is running, the ECU warning lamp will begin to flash out the codes (wait for the initial boot up check that lights up all the warning lamps). Each fault code is repeated 3 times and then it moves onto the next one (if any). Once at the end of the logged fault code list, it will go to the beginning again, giving you plenty of time to note down the code (on each fault code shown, there will be a pause before it shows the next one). It will carry on like this for ever until you remove the key from the ignition or the battery runs flat. To clear the codes simply turn on and off the ignition 30 times with a 5 second 'on' gap in-between (time allowed for self check to finish).

For example, lets say there was a ECU warning lamp noted and the owner wanted to read what fault code was logged. The owner has already put the wire link in place and switched on the ignition. A flash is indicated by an astrix (*) and a pause is shown as a dash (-).

*-** (1 flash, 1 pause and another 2 flashes = code 12)
*-** (12)
*-** (12)

**-*** (23)
**-*** (23)
**-*** (23)

*-** (12)
*-** (12)
*-** (12)

**-*** (23)
**-*** (23)
**-*** (23)

*-** (12)
*-** (12)
*-** (12)

Owner then takes the key out of the ignition.

Lets go through what has happened. After switching on the ignition, the dash board lights up showing the self check, after that all the lights go out and the ECU warning light begins to flash the error code. The first code is 12 (one flash, a pause and then two flashes). This is repeated 3 times.

Code 12 at this moment means initiation of diagnosis. After that came code 23, which means 'Knock control module', i.e. there is most likely a faulty unit or a bad connection at its plug. After code 23 has been repeated 3 times it flashes a separator code, which is code 12.

It flashes this 3 times before going to the next fault code. The next code is code 23 again which means its finished showing the logged fault codes and has started back at the beginning. The owner can switch off the ignition. A healthy car should display a stream of code 12's (note: some ECU systems will also show code 31 or 19 because it doesn't see the engine turning over, this is not a fault because you haven't started the car). Simply start the car and read the fault codes again. To find out what fuel system you have, read the "Fault Code Chart" below.

The Simtec 56.5 fuel systems use a 4 digit flash code. The engine ideally needs to be running when reading the fault codes, otherwise it may show incorrect fault codes.

Once the wire link is in place and the engine is running, the ECU warning lamp will begin to flash out the codes (wait for the initial boot up check that lights up all the warning lamps). Each fault code is repeated 3 times and then it moves onto the next one (if any). Once at the end of the logged fault code list, it will go to the beginning again, giving you plenty of time to note down the code (on each fault code shown, there will be a pause before it shows the next one). It will carry on like this for ever until you remove the key from the ignition or the battery runs flat. To clear the codes simply turn on and off the ignition 30 times with a 5 second 'on' gap in-between (time allowed for self check to finish).

For example, lets say there was a ECU warning lamp noted and the owner wanted to read what fault code was logged. The owner has already put the wire link in place and switched on the ignition. A flash is indicated by an astrix (*) and a pause is shown as a dash (-).

Note that 10 flashes = number 0.

**********-*-*-***** (10 flashs, 1 pause, 1 flash, 1 pause , 1 flash, 1 pause and 5 flashes = P code 0115)
**********-*-*-***** (0115)
**********-*-*-***** (0115)

**********-***-***-********** (0330)
**********-***-***-********** (0330)
**********-***-***-********** (0330)

**********-*-*-***** (0115)
**********-*-*-***** (0115)
**********-*-*-***** (0115)

**********-***-***-********** (0330)
**********-***-***-********** (0330)
**********-***-***-********** (0330)

Owner then takes the key out of the ignition.

Lets go through what has happened. After switching on the ignition, the dash board lights up showing the self check, after that all the lights go out and the ECU warning light begins to flash the error code. There is no initiation flash code, the ECU will flash out the fault codes straight away. This is repeated 3 times.

P code 0115 means 'Intake manifold pressure sensor', i.e. there is most likely a faulty unit or a bad connection at its plug. It flashes this 3 times before going to the next fault code. Remember to show a 0 number it flashes the ECU warning lamp 10 times.

The next code is code 0330 which means 'Knock sensor 2' which suggests the engine has two knock sensors on it, hence the 2nd one is faulty or has bad connections. Again this flashes the same fault P code 3 times before moving onto the next. In this case it there are no more faults and the P codes repeat themselves.

The owner can switch off the ignition. A healthy car should display no fault codes what so ever, i.e. the ECU warning lamp never comes on or flashes. Some ECU systems will also show code 0335 because it doesn't see the engine turning over, this is not a fault because you haven't started the car. Simply start the engine and read the fault codes again. If 0335 shows again while the engine is on, then its a fault. To find out what fuel system you have, read the "Fault Code Chart" below.

Over the years there have been many different models in the Vauxhall range and hence there are different ECU systems. With each different system there are (usually) different fault codes. Listed below are the fault codes, search for the model of your car and click on the link to view the fault codes.

This list is by no means complete and still need gaps filling in and some corrections. If you know more about other engine systems or can correct anything below, please e-mail us at ecu_codes@topbuzz.co.uk. Thank you.

Fault: