yes sorry, Corsa C

ah ok, it was showing pounds on the diagnostic screen (I'm sure it was!) will check tomorrow.

is the fact that the readings hardly move when flat out in third giving some clue do you think?

seems a normal MAF reading in grammes is 3.5 grammes per second which is 0.00771618 pounds per second = 0.46 pounds per minute. No idea why my can tool is measuring in pounds. My MAF is showing 0.00 pounds at idle instead of 0.46 pounds. When the car returns I will check it disconnected - presume if it shows 0.00 ie no change, it needs a new MAF?

The post catalyst O2 sensor, also known as B1S2 should give a flat reading of around 0.45v. It's purpose is to tell the ECM that catalyst has done it's job.

The pre catatyst O2 sensor B1S1 should switch between rich and lean (0.1 - 0.9v). Short term fuel trim is a report of how the engine is running. Long term fuel trim is a report of the corrective action taken by the ECM in response to STFT.

The job of the various sensors on the engine management system is to provide information to the ECM, which then runs the engine accordingly. Fault codes do not mean that a sensor is at fault (although of course it is possible that one is), it means that the information reported by the sensor is outside of the ECM parameters. The cause of the fault needs to be determined, often by more conventional and old fashioned means.

For example a loss of airflow would be reported by the MAF sensor, but the ECM is not able to tell the difference between a reported loss of airflow because of a faulty MAF, and an actual loss of airflow caused by something else. Because the MAF is reporting the fault, a MAF related fault code would be stored, even though there may be nothing wrong with the MAF. To help you to diagnose faults on a modern car you must think the way a computer works. The don't understand logic, common sense or reason. They know what to expect and if they don't get it they shout.

What make o2 sensor did you fit?genuine or some happy shopper variety?

Bosch should be fine if the part no matches the original.If the car drives fine with no major issues,then there is probably nothing wrong with it.

hmmm ,mystery deepens - Eurocarparts sold me a 02 sensor with Bosch part number 0258 006 171 for my 06 Corsa C 1.2 twinport.

On a seperate lambada sensor sales site the front 02 sensor for an 06 plate is showing as 0258 006 501 . The part number for the one ECP sold me is applicable to Corsa C to 04 whereas mines an 06.

Eurocarparts DID sell me the correct one as bought up by their number plate search but they dont list Bosch part numbers - I got that off the box

Maybe this is the prob then....

thanks railroad.. so thats negative probe to earth and positive to one of the two white wires with multimeter set to 20V and ign on? - either of those 2 white wires?. Then test OHMs on the 2 white wires on the sensor...

what about the part number for the 02 sensor. ECP have definately given me the wrong one (front sensor) according to bosch parts manual.

Cars running with maximum fuel trim often have an air leak on the induction system or an exhaust leak, Failing that I'd be checking fuel pressure.

RR thanks for such a comprehensive answer. It kind of tells me how it works but not whats likely to be wrong but sure I'll get there in the end!

Returned front 02 to ECP and they are giving me the right part. Someone else told me the different part numbers just relate to cable lengths, a 02 supplier told me the two sensors are the same also that Bosch sensors dont work well on these engines and I should use Delphi which is OE.

Anyway, put old original front 02 sensor back on this morning and ran the car. The STFT are drastically better as a result but engine idles more unevenly and p141 still on. Theres also a P170 showing on the VX setting on the scan tool whereas with OBD11 it doesnt show.

I will test the rear 02 sensor in a minute! Just a thought, I do have this £200 scanner so not a £9.99 job from Halfords and it gives me all live data - is there anything I should be looking for? It shows the front and rear 02 sensor readings with engine running. The front changes every second more or less up and down (400 to 900 ish) but not that defined and the rear just hovers up and down around 460. Is it still possible the rear sensor is faulty given these readings?

Also relating to the other poster - I have sprayed carb cleaner all around and nothing. I even sprayed it around brake servo still nothing so presume there are no air leaks?

turning into a nightmare now!

oops those OHM readings should be .10. Car seems now sorted. Fuel trims are looking good I think STFT around 0.00. LTFT around 3. EML is still on but presume thats due to the duff rear sensor which I will replace. It will be interesting to see if replacing it makes the car go better or whether its just there to record the CAT condition. Car goes ok now anyway.

The fix seems to have been the front sensor. Since Nov I've used 3. Putting all 3 back in today I could see what worked best.

first bosch replacement (wrong part number) both fuel trims 25%
second bosch replacement (correct part number) both fuel trims good
original sensor mid way between the above 2.

so it seems the 2 bosch sensors are different sensors unless theres another reason one works well and the other doesnt.

Thanks for all advice, I hope the thread will help someone else.

Hi RR, that front 02 sensor reading I gave of 400 - 900 was for the original sensor where the engine wasnt running properly. I will take another close look at the front sensor reading now the new sensor is on also will keep an eye of the LTFT. . What sort of readings should I be expecting for the new sensor to show the engine is fueling correctly?

RR, ok heres the front 02 readings, car nice and hot....

150-112-735-750-125-740-115-110-740-765-500-125-740-115-110-740-745-125-110-720-755-355-110-695-750-120-135-710-730-110-125-710-705-105-115-760-735-110-120-745-745-105-115

decimal point in front of all those. Numbers changed every second.

LTFT is now reading +1.6

what do you think?

PS fitting new rear 02 on fri. Does it being faulty affect fueling at all?

Edited by Matt@ on 03/02/2016 at 19:57

must admit, I've learnt loads from this - thanks so much for all your info RR - much appreciated. Hopefully new 02 sensor near to the CAT on Fri will sort the light as its a P0141 after all.

Edited by Matt@ on 03/02/2016 at 21:30

Fuel pump values for this are engine running.