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[terry paget]

As above. This car has 2 lambda sensors, before and after catalyst. Which do I replace?

[JamesV6CDX]

Well, the one before the cat is the only one that controls fuelling. The one after the cat only measures efficiency, and therefore has no bearing on emissions.

But with that said, why do you think you need to change the lambda sensor? The readings could be caused by a host of other things, a random example being a slight misfire, a blow at the downpipe join, etc etc...... 

[terry paget]

Thanks for quick response, James.
The MOT emissions tester passed it on CO <0.3% and HC <200ppm but failed it on lambda at 1.31, so I presumed the fault was in the lambda sensor. I see over the last 18 months I have had a persistent EML fault of P0171 (fuel trim bank 1) and I have twice changed the first lambda sensor for old spares. Do these additional facts point to faulty first lambda sensor?

[Kevin Wood]

Just because the lambda value is out, doesn't mean it's the lambda sensor that has failed. The fact is that the fuel mixture is wrong (lean, by the look of things - sue it isn't 1.031 - 1.31 is MILES out). SO, anything that can give a lean mixture is possible - induction air leak, bad MAF sensor, misfire (unlikely, as it passed CO and HC) and so on.

Need some live data from it, ideally.

But yes, it's only the front Lambda sensors that have any bearing on emissions.

[terry paget]

Correction - reading is 1.131. Car runs well, no misfiring, stalling. Will check for induction leak between MAF sensor and engine, and check if I have a spare MAF sensor. Would it be the same as a 2.5 MAF?

[terry paget]

Obviously not the same as a 2.5. But could be the same as a 2.0. I will check in the morning in my parts heap.

[terry paget]

Not the same as a 2.0, I've got one here. 2.2 MAF is part no. 190803, 2.0 is 359806.
I see no obvious air leaks between MAF sensor and engine.

[flyer 0712]

My previous 2.2 was a bitch to get through the m.o.t each time on the same thing....I found that a very friendly tester put himself out by giving it a long time on the test and did it several times before it eventually passed,,,he said that 2,2 sometimes need more time ticking over and retrying a few times before the reading comes down,,needless to say i took it to him after that simply because he put him self out  ..

[EMD]

Or ....




And rag it 

[terry paget]

Thanks EMD. Are you suggesting the catalyst is tired, and in need of reinvigorating? The catalyst is a Timken aftermarket item. I was prepared for its failure last year, but it passed. This year  the CO and HC readings were fine, so I presumed the catalyst was all right. In my simple way, I suppposed that an out of spec lambda reading meant that the lambda sensor was sick. I have known them fail before. They are easier and cheaper to replace than catalysts.
Keep the advice coming chaps! I have a problem.

[Kevin Wood]

Cat is working fine looking at those figures. It's a mixture control problem.

[Marks DTM Calib]

Kevin is correct, the Cat is good, its the mixture control which is out.

Something is affecting it be it an engine hat is not getting to temperature (e.g. stat stuck open) so closed loop control is not working or something is overriding or affecting the closed loop control value.

[terry paget]

Thanks to all for wise advice. Is it your view that the lambda sensor is unlikely to be the culprit?

[Nick W]

Thanks to all for wise advice. Is it your view that the lambda sensor is unlikely to be the culprit?

The lambda sensor is doing exactly what it was designed to; reporting an issue further up.

[terry paget]

Thanks to all for wise advice. Is it your view that the lambda sensor is unlikely to be the culprit?

The lambda sensor is doing exactly what it was designed to; reporting an issue further up.

Thanks Nick. My problem is getting this car through its MOT. If car lambda sensor thought mixture was too lean would it not report this to the ECU, which would then correct matters? If the lambda sensor was faulty, it would report duff information to the ECU, resulting in an incorrect mixture. Then the MOT tester's kit would report faulty mixture, throw up a faulty lambda reading and fail my car.