MOT fail on Lambda reading...

[iPond]

Took my old scooby down for an Mot today an it failed on the Lambda reading. Never had any issues on previous mot's and haven't made any changes to the car recently. The car hasn't been used for the last 7-8 months but I have been starting her up.

Reading print out below;


Has the Lambda had it? If so, how do I know which one?

Few comments online are suggesting it isn't always the Lambda causing the issue? Any help appreciated!

[Ginola]

Was the car hot (Ie had it had good drive to the test centre or did the tester just start it up and run the test cold)

It's quite possible it will pass the next time its tested if the thing is hot, the lambda reading on the MOT test is a calculated one and not an actual so I would guess there is nothing wrong with your existing Lamda sensor unless your ECU is throwing CEL lights everywhere.

[iPond]

I drove it for at least 30mins to the garage but it wasn't tested for another 4 hours so would have cooled. Did wonder if this could be the case so asked him to run another emission test after getting her nice an hot 1st thing Monday, luckily I know the guy so he's gonna try his before a re-test to make sure.

There's no warning lights but did notice it seemed to be idling a little higher than i think it used to.

[munk]

Sounds like a good warm.up may do the trick if youve had no problems before...fingers crossed.

[Scott.T]

The cat is clearly hot enough to work as your emissions are nice and low, so I would expect the lambda to be hot enough too.
The lambda do have a heating element which shoud get them in the working range before the catalyst.

Lambda can be a bit of a consumable I'm afraid.
It will be the front (expensive one) if it has failed as the rear one is just there to check the efficiency of catalyst and only compares to what it reads from the front lambda.

There is a possibility there is an air leak somewhere, perhaps in the exhaust, as your lambda reading is very high which means it reads a lean mixture.
If it were as lean as it's reporting it wouldn't be running.

It should be running Lambda 1, which = 14.7AFR
Your reading is Lambda 2.9 !!, which = 42.6AFR !!!!!

They start to run lumpy at idle around 15.2AFR, so your well away from even running if that's a true figure !!!!!

P.S The Lambda sensor is nothing more then a kind of battery cell that generates voltage based on oxygen content. When it sniffs oxygen it generates a small voltage from a chemical reaction. This voltage is normally 0.5-1V during the ECU control of the fuel mixture. A car running a closed loop 14.7AFR fuel target normally oscillates between 0.5V and 1V.

[Scott.T]

Here you go, just found this which confirms what I said above :

"If the fault is a high lambda reading, with a high O2 reading but with normal CO and HC readings, the most common cause is post combustion air leaks. A small exhaust system air leak will leak air into the exhaust before any blows can be detected. Only a small volume of clean air introduced into the exhaust will cause a car to fail the MOT test. "

[Scott.T]

Something else to note : If you have a huge 5" tailpipe these can give bad AFR readings at idle and light cruise (I hate these when trying to map in new injectors), due to the volume of air and turbulent nature of the exiting fumes.
So the MOT may need to ensure the probe I well into the pipe.

[iPond]

So it sounds like it's not the temp but could be either an air leak or a faulty sensor?

Seen a 2nd hand Lambda for £60 and wondering if I shud nab it quick.

[iPond]

Well she was adequately warmed before a second emissions test this morning, still no joy. Front Lambda replaced, little improvement but not enough. Inspected for exhaust leaks, nothing obvious found.

At this point it was decided to refer to JPP as they were nice an close and a simple MOT was starting to spiral. Saj agreed to take it an get one of his guys to check it out.

1st suggestion was that the missing downpipe cat was causing the issue. I challenged that surely this wud hav affected the previous MOTs as nothing has changed. Also why wud the co2 and HC readings be OK as pointed out above. He agreed this was odd so wud continue without adding the cat back in.

Maf removed and cleaned, idle control valve changed to resolve high idling issue. Further detailed inspection revealed a split in an intake pipe... so now the inlet manifold has to come off, gaskets replaced etc. in order to fix the split. Hopefully this will all be finished off tomorrow an she gets a pass.

Fingers crossed! It's been an expensive MOT so far!

[Banstead Stig]

Fingers crossed!

[iPond]

Quick update on the above...

Got the call late on Tuesday to advise she had finally passed and was ready to be picked up. Lucky, as I was away on a business trip for the rest of the week and cudnt/didn't fancy leaving her there until the following Monday. Drive home was very pleasant, felt nice and smooth and idle issue all resolved.

Now I need to get my rear end in gear and get round to taking some pics and putting up a for sale thread! Can't keep 2 scoobs and it's only going to cost me not using her.