Lexia C2 P0131 fault and check engine light

[punkieys17]

The fault seems to relate to the oxygen sensor (replaced) or the wiring. Has anyone experienced this - it appears the car is running too rich and flagging a fault. Any help on the diagnosis greatly appreciated, C2 2004 1.1 SX cheers

[Mandrake]

P0131 means the oxygen sensor voltage is staying low all the time, which would normally indicate a lean condition if the oxygen sensor was working. On the other hand if the ECU was receiving a low voltage erroneously (faulty oxygen sensor, oxygen sensor heater not working, shorted wiring etc) then it would try to richen the mixture to compensate for the perceived lean condition.

What other symptoms make you think its running rich ?

If you look at the oxygen sensor voltage with the Lexia (under parameters measurement) what does it say when idling ? Give it a couple of minutes running to let it warm up.

You should see it switching back and forth between 0.1 volts and 0.9 volts roughly once per second or twice per second at a steady 2000 rpm.

If its still reading low all the time there are a few things you can try. In case the oxygen sensor is working and the engine is actually running lean (vacuum leak for example) try injecting some propane/butane carefully into the air filter box for a few seconds, it should swing hard to about 0.9v to indicate rich. If this happens you know right away that your oxygen sensor and wiring are ok and that the ECU is reading the sensors voltage ok and that you really do have a lean condition or a false lean from a misfire.

If you still see near zero volts try unplugging the oxygen sensor, use a pin or similar in the active signal terminal on the harness side (refer to wiring diagrams to find the correct one, or use trial and error) then touch one finger onto the pin inserted into the terminal and your other hand onto the battery positive post. This should be enough to make the oxygen sensor reading on the Lexia swing to at least one volt.

If that works the ECU and harness side wiring to the oxygen sensor should be ok. That should be enough to get you started...let me know what results you get.

I've pasted this link before elsewhere but ScannerDanners diagnostic videos on youtube are outstanding and there are several that cover diagnosing oxygen sensor problems, you can find them here:

http://www.youtube.com/user/ScannerDanner" onclick="window.open(this.href);return false;

Useful both for background understanding so you know what to do and why you're doing it, but also very practical as he demonstrates the practical side of it step by step...a lot of what I've learnt about troubleshooting engine running problems have come from studying his videos in detail and then putting it into practice on problems on my car

Edit: check this one of his videos in particular, its basically what I was describing: