Diagnose Your ECU Faults - Older OBD system

That dreaded 'K' light keep coming on and want to know what's wrong? This ECU fault diagnosis refers to Peugeot petrol engines, not Diesel, but I will update this as soon as possible to include the new HDi Diesel fault codes (previous Diesel units to the HDi do not have an ECU).

Learn more about ECUs and their function here

The ECU constantly monitors elements of the engine and controls various parts of it to make sure it runs efficiently. If a fault should develop in any part of the engine being monitored, the check engine light (sometimes referred to as the 'k' light) will illuminate on your dashboard. The ECU will record the fault in the memory as a number and this can be read out by your garage with sophisticated equipment or with a budget DIY reader. In many cases the fault codes can be read out just with a piece of wire - see below!

The following is taken from a 1994 Peugeot 1.8 (8 valve) but the testing and diagnosis is the same with other Peugeot vehicles (and Citroen) using this same diagnostic system.

On this model the diagnostic plug (normally green) is clipped in just behind the ECU enclosure (arrowed). On other models it can be situated inside the main electrical boxes, normally near the suspension turret.

The green diagnostic plug is arrowed here. It is clipped into a socket with no electrical connections just to keep it secured. To remove simply depress the thin metal securing clip with your thumb while pulling out. You might have to jiggle it a little.

Once you've pulled the diagnostic plug out you'll see it has 2 connections.  Note the position of Terminal 1.

To read your codes out you need to monitor the ECU light on your dashboard and how many times it flashes. Turn your ignition switch to ON (II). Now connect a wire into Terminal 1 and then touch the other end to vehicle earth (-) for between 3 and 5 seconds. You can do this with a decent single core cable with a spade connector crimped either end or even bare, providing it makes a good contact. After removing the wire from earth the ECU light on the dash will start to flash to indicate any fault codes stored in the memory.

Each code output will be 2 digits as detailed in the table below. The first code will be 12 to signify the start of the test. This will be shown by 1 flash (1), a short pause and then two flashes (2). After this the next two digits will represent the first fault code. To find the next fault code (if there are anymore) you need to connect the wire to vehicle earth(-) again for 3-5 seconds and disconnect. You will now recieve 2 sets of flashes, separated by a short pause, to signify the next fault code. Continue this procedure for more fault codes until you get code 11. Code 11 signifies the end of the test and there are no more fault codes stored. Now switch the ignition off.

Code	Code Meaning	Action to Take
11	End of test
12	Start of test
13	Air Temp sensor	Refer to Test Notes 3
14	Coolant Temp sensor	Refer to Test Notes 2
15	Fuel pump relay	Test pump circuitry
21	Throttle position sensor/idle	Refer to Test Notes 4/5
22/23	Idle speed control valve	Refer to Test Notes 10
27	Vehicle speed sensor	Test for speed signal to ECU
31	Lambda system control	Refer to Test Notes 9
32	Lambda system control	Refer to Test Notes 9
33	MAF/throttle switch/MAP sensor	Refer to Test Notes 7/4/6
34	Carbon filter solenoid valve
35	Throttle position sensor/full load	Refer to Test Notes 4/5
36	Lambda sensor heating control
41	Crank Angle Sensor
42	Injector valve control	Test for C.A.S. signal to ECU 43
43	Anti-knock control
44	Knock sensor No.1
45	Ignition coil control No.1
46	Turbo pressure valve
47	Turbo pressure regulation
51	Lambda sensor	Refer to Test Notes 9
52	Lambda system control	Refer to Test Notes 9
53	Battery voltage
54	ECU and anti-knock module
55	CO mixture potentiometer
56	Coded anti-theft
57	Ignition coil control No.2
62	Knock sensor No.2
63	Lambda sensor No.2	Refer to Test Notes 9
64	Mixture control B	Refer to Test Notes 9
65	Cylinder sensor
71	Inj. valve 1
72	Inj. valve 2
73	Inj. valve 3
74	Inj. valve 4
75	Inj. valve 5
76	Inj. valve 6
91	Inj. valve control

Clearing Fault Codes

Once you've effected a repair you should clear the fault codes, then check for fault codes again after some test driving to see if they re-occur.

To clear the codes all you need to do is reconnect the wire to vehicle earth for 10 seconds after code 11 (test end) is flashed.