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[Kevin Wood]

You could indeed have a faulty lambda sensor that the ECU has detected and, in response, gone open loop but many other things could cause it to go open loop and Lambda sensor failure is not one of the more common, IMHO.

Other possibilities:
Air leak causes the fuel trims to go out of range
Temperature sensor fault causes the ECU to believe the engine is not hot enough for reliable closed loop running
MAF sensor failure causes the fuel trims to go out of range.
Low fuel pressure or tired injector causes fuel trims to go out of range
Air leak into the exhaust causes false lambda sensor reading
Air leak into the exhaust causes the MOT tester's lambda reading to be inaccurate

Is the engine light on, and have you checked the engine ECU for fault codes?

[terry paget]

You could indeed have a faulty lambda sensor that the ECU has detected and, in response, gone open loop but many other things could cause it to go open loop and Lambda sensor failure is not one of the more common, IMHO.

Other possibilities:
Air leak causes the fuel trims to go out of range
Temperature sensor fault causes the ECU to believe the engine is not hot enough for reliable closed loop running
MAF sensor failure causes the fuel trims to go out of range.
Low fuel pressure or tired injector causes fuel trims to go out of range
Air leak into the exhaust causes false lambda sensor reading
Air leak into the exhaust causes the MOT tester's lambda reading to be inaccurate

Is the engine light on, and have you checked the engine ECU for fault codes?

Engine light is not on at the moment, but as stated in reply#2, P0171 fuel trim bank 1 is a persistent light on this car, and twice changing upper lambda sensor has not eliminated it In 2013 MOT lambda value was 1.023, in 2014 it was 1.025.Thanks for all your suggestions, I will cheack all I can on Saturday.

[Kevin Wood]

OK. Sounds like something has taken the fuel trim out of range and caused it to go open loop. 

Can you read live data from it or just the fault codes?

[terry paget]

OK. Sounds like something has taken the fuel trim out of range and caused it to go open loop. 

Can you read live data from it or just the fault codes?

Cannot read live data, only fault codes.
Cannot detect any inlet side air leaks.
Held hand over exhaust, engine running, no sound of leak; removed hand, whoosh! No exhaust leak.
Engine temp sensor? possible. Fuel pressure low, injector dodgy? Possible again. But car runs so well, idles well. Doesn't seem to lack power.
MAF sensor faulty? Could be.
Lambda sensor fault? Could be. But I would expect a fault code. I would expect fault codes for the above too.
Would live data tell me straight away? The local Vauxhall dealer might give me a Tech2 readout as a favour if it would be straightforward and conclusive.

[Kevin Wood]

The live data would need a certain degree of interpretation, so you might struggle, depending on whether the guy knows his onions.

The £60 they would probably charge you might be better spent on an OBDII reader that can do live data, perhaps? Might be a good investment.

I don't suppose it's suffering the cracked exhaust manifold that's common on 2.2s? That might give enough of an air leak to upset things.

Failing that, can you borrow a known good MAF and see if that clears the P0171 code?

[terry paget]

The live data would need a certain degree of interpretation, so you might struggle, depending on whether the guy knows his onions.

The £60 they would probably charge you might be better spent on an OBDII reader that can do live data, perhaps? Might be a good investment.

I don't suppose it's suffering the cracked exhaust manifold that's common on 2.2s? That might give enough of an air leak to upset things.

Failing that, can you borrow a known good MAF and see if that clears the P0171 code?

[terry paget]

The live data would need a certain degree of interpretation, so you might struggle, depending on whether the guy knows his onions.

The £60 they would probably charge you might be better spent on an OBDII reader that can do live data, perhaps? Might be a good investment.

I don't suppose it's suffering the cracked exhaust manifold that's common on 2.2s? That might give enough of an air leak to upset things.

Failing that, can you borrow a known good MAF and see if that clears the P0171 code?

I don't hear crackling from exhaust manifold, unlike some other Omega fours, so don't thnk it's cracked.
My son Jonny also has a 2.2 so I could swop MAFs, trouble is he lives 30 miles away, and as code P0171 is not displaying I am not sure what good that would do me.
Is this the sort of reader you had in mind?
http://www.amazon.co.uk/Blusmart%C2%AE-OM580-Diagnostic-Compliant-European/dp/B00VV3U16I/ref=sr_1_3?ie=UTF8&qid=1435403329&sr=8-3&keywords=obd+ii+scanner

[RobG]

Got a Maxscan 509 (Autel) you can use if you`re passing Terry

[terry paget]

Got a Maxscan 509 (Autel) you can use if you`re passing Terry

Thanks Rob. PM sent

[terry paget]

Thanks Rob. Would this morning be convenient?

[RobG]

Thanks Rob. Would this morning be convenient?

Only just saw this. thought you were popping over last night?

[terry paget]

Thanks Rob. I got your message last night too late to make it, so I thought I might come today. It's late to come tonight, so if I am still stuck next weekend I will fix a time early. Thanks again for the offer.

[terry paget]

Thanks for all the advice. Could detect no leaks inlet or exhaust, so decided to change lambda sensor. Bought one, Halfords £41, fitted Monday, rang to arrange retest, tester closed for a week. Wednesday son Jonny rang, his 2.2 losing water, see different thread. I left him MOT failure car, collected leaky car, brought home and diagnosed failed head gasket. Yesterday family gathered here, Jonny arriving in MOT failure car, now with EML on again, P0171 fuel trim bank 1. as before. Oh dear, fault still present. I then swopped MAF sensors. Submitted car for retest today, passed easily.
I reckon the fault was the MAF sensor, though engine ran well enough. Jonny's car did not like it though, running very badly. All's well that ends well! Thanks again.

[Marks DTM Calib]

We did say the MAF......

....fuel trim is nothing to do with the O2 sensor.

[terry paget]

We did say the MAF......

....fuel trim is nothing to do with the O2 sensor.

You did indeed. I suspected if it were MAF sensor there would be other symptoms, like bad running, stalling, misfiring. I was wrong. Funny thing is that putting the duff MAF sensor in the other car did cause hesitant running and stalling.
Anyway, that car has passed its MOT. Tomorrow I submit the ex-plod 3.2 for MOT. Next week would have been the other 2.2, but now it needs a new head gasket, Who needs hobbies with a fleet of Omegas to occupy my time?