TERMINOLOGY:
ECU: the Engine Control Unit, which is a computer located on the right of the driver's side footwell. this computer controls fuel delivery to the engine.
EZK: a smaller ECU located on the passenger side firewall that controls spark timing for the engine.

Right, so you've go the dreaded lambda light illuminated on your dash. The car may seem to be running fine, it may be running rough. No matter, the light is telling you the ECU or EZK has detected a fault. What we're going to do is find out what the computers think is wrong. The first step is to open the bonnet:

The arrow points to the diagnostic box. If you take a closer look, it looks like this:

Pull the lid off the box, and you're presented with six sockets, a diagnostic probe, an LED and a button:

With the ignition turned off, insert the probe into socket 2 (to read codes from the fuel ECU) or socket 6 (to read codes from the EZK). Turn the ignition to key position 2 without starting the engine. Press down the button on the diagnostic box for about 2 seconds, and the LED on the box should flash out a three-digit code, i.e 3 flashes, space, 2 flashes, space, 1 flash would be code 3-2-1. After reading the code, depress the button again for 2 seconds to see if further codes are stored. If the same code repeats, there are no further fault codes stored.

It is also worth noting that not all fault codes cause the lambda light to illuminate. If you're having running difficulties, it's worth checking to see if there are any stored codes which could point you the right way.

Below is a list of all the fault codes that can be stored:

ECU:
1-1-1 - No faults
1-1-2 - ECU internal fault
1-1-3 - Lambda control - engine running too rich/lean
1-2-1 - Mass air flow meter signal faulty or missing
1-2-2 - Air temperature sensor signal faulty or missing
1-2-3 - Coolant temperature sensor signal missing or faulty
1-3-1 - RPM signal from EZK missing
1-3-2 - Battery voltage too low or high
1-3-3 - Throttle position sensor idle setting faulty
1-5-4 - Exhaust Gas Recirculation flow too high
2-1-2 - 02 (lambda) sensor signal missing or faulty
2-1-3 - Throttle position sensor full load setting faulty
2-2-1 - Lambda control - engine running too lean at part load
2-2-2 - Main relay faut
2-2-3 - Idle Air Control valve signal missing
2-3-1 - Lambda control - too lean or too rich at part load
2-3-2 - Lambda control - too lean or too rich at idle
2-3-3 - Idle air flow outside control range
3-1-1 - Speedometer signal to ECU missing
3-1-2 - Knock sensor signal (from EZK) missing
3-2-1 - Cold start injector signal missing or faulty
3-2-2 - Mass Air Flow sensor burn-off signal missing or faulty (MAF wiring fault)
3-4-4 - Exhaust Gas Recirculation temperature signal missing or faulty

EZK
1-1-1 - No faults
1-4-2 - EZK internal fault
1-4-3 - Knock sensor signal missing or faulty
1-4-4 - Load signal from ECU missing
2-1-4 - RPM sensor signal intermittently missing
2-2-4 - Coolant temperature sensor signal missing or faulty
2-3-4 - Throttle position sensor idle signal faulty

Once you've repaired the problem or to check if the problem is still present after repairs, you'll need to clear the codes from the ECU/EZK. This is accomplished by inserting the probe into the appropriate socket and read the codes as detailed above. Once the codes have read themselves out, press the button for more than 5 seconds. After 3 seconds, the LED should light. Press the button again for more than 5 seconds. the codes should now be erased. You can then repeat the procedure for reading codes and you should read '1-1-1' for 'no faults stored'.

I hope this post is useful and keeps everybody on the road! If anybody's got any additions or amendments to what I've written, don't hesitate to contact me or post them up.

The instructions in this tutorial will work in the following models / years:

- 1989, 1990, 1991, 1992 and 1993 Volvo 240, 242, 244 and 245