Lambda probe condition

Up to a point - the cat itself will mask minor fuelling faults... Usually total failure will result in the engine management light coming on and poor driveability.

Probably not, unless you can totally isolate any other causes of emissions anomolies. So many other factors can affect such a reading:

condition of the air filter condition of the spark plugs quality of fuel engine wear engine problems - such as valves sticking on a Zetec the calibration of the individual test machine - they can vary very slightly

You could visually inspect it, but you wouldn't be able to see the actual probe anyway.

The only test that would show something would be an oscilliscope viewing of it running. You should see regular changes from about 100mv to 800mv, use 500mv as the centre line.

If it doesn't move much off the centre line then it's probably on it's way out. If it sits at 1v then it is most likely the engine is running badly (I can't recall if it's weak or rich right now). If it reads 0v then it could also be a running problem (opposite to above), or its gone open circuit.

If I could rephrase the question a little. If the results of the emissions test were well within limits, could I assume that the Lambda probe was working fine?

In message , chris writes

Thanks for that Chris.

Yes.

Almost certainly.

I should have asked it that way in the first place, shouldn't I?

;)

Cheers Andy, I got there in the end :)

It was simple wasn't it? ;-)

Yes :-)

^^^^^^^^^^^^^^^^^^^^^^^^

My Mondeo failed it's test last week on emissions. I changed the plugs and airfilter, took it for a thrash and it scraped through. However, when I was changing the cambelt end plug, it was very oily on the electrodes, and I could see oil in the cylinder when a torch was shined down. Sticking valves?

Any advice appreciated! Mike

No, that's more likely worn valve guide seals, or worn/stuck piston rings

However, if you're sure the plugs were removed without any fiddling arond, this could be so, otherwise it might just tbe leaking rocker cover seals.

The Escort had trouble getting a pass on the emissions a couple of years ago. I changed all of the filters, the garage replaced the cat and the last thing changed was the lambda sensor (which turned out to be the fault). It seems they aren't very prone to fault and there are so many more basic things that'll go wrong that its well down the list of diagnostics.

If assuming there was a reason for the question and the only one I can imagine is that fuel economy is down.

If his emissions are a pass and fuel economy is down then it isn't likely to be the lambda. An air leak somewhere would do it though.

Just pondering

Warwick

Indeed. I went on a Ford trainign course a few years ago, and the instructor told us that he'd been given the task of testing some 3000 Lambda sensors that had been returned as warranty repairs. Only one was actually faulty.

It should be very low down on the list of possibilities. However, you should also look at other information too, such as likely causes on a particular vehicle.

later Escorts, for example, had the block connector mounted upside down behind the radiator. As such they were prone to filling with water. Simply drsining the water and cleaning the contacts was enough to effect a repair.

On a Fiat, you could quite easily suspect the sensor in the first instance.

Doubt that, an air leak is likely to cause poor emissions too.

There's plenty of food for thought here ;-)

The message from snipped-for-privacy@spamcop.net (Andy Hewitt) contains these words:

The only two sensors I've replaced have both been on Fiats.

Yes, they are a very common fault. Very often you'll find they bring on the warning light but don't store a fault code.

If the CAT's handling it and the lambda is OK wouldn't the emissions be OK from a rich runner?

I was thinking of a small leak after the airflow sensor on the intake rather than a big hole in it.

Warwick

I see, maybe. I'd have to test that to find out though, it could be different for each model of vehicle. It might be possibe I guess, but the leak would have to be very small.

Had the cat really been killed by rich running due to duff lambda sensor. Or were they just wandering around in the dark replacing suff willy-nilly.

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