1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions

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Sun 6 Feb 2011 15:00
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - nickphonics18

We had the MOT done on friday and it has failed on emissions!

They said they were 80% sure its the lamber sensor. They replaced it and it hasnt solved the problem.

On tuesday its going to VW dealer for a diagnostic test to source the issue.

Here are the readings after the lamber sensor was replaced. Does anyone know the issue by looking at these....

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Fast idle test
Engine speed - 2654rpm PASS
CO level - 0.95% FAIL
HC level - 323ppm FAIL
Lamdba - 0.96 FAIL

Scond Fast idle test
Engine speed - 2619rpm PASS
CO level - 0.62% FAIL
HC level - 227ppm FAIL
Lamdba - 0.98 PASS

Natural Idle test
Engine speed - 866rpm PASS
CO level - 0.19% PASS

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Sun 6 Feb 2011 15:24
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - unthrottled

Nick-There's nothing there to suggest anything fundamentally wrong with the engine. These are good 'engine out' emisions-ie what a healthy engine should emit without a catalytic converter. The engine is running slightly rich, although not worryingly so (an engine will alternate between slightly rich, and slightly lean ie lambda.1.0) to maintain effective catylitic converter operation.

Looking at these numbers, I'm not sure your catalytic converter is working at all. With a slightly rich mixture, I'd expect there to be some CO (the catalytic converter can't make oxygen out of nothing!), but very little HC. Could you post the NOx numbers? If the cat is working, these should be very low. If the NOx numbers are high ie fail-the cat is faulty.

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Sun 6 Feb 2011 19:05
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - nickphonics18

Thanks for a quick reply! :)

I have looked through the emission test print out and i cant see any mention of NOx number on there?

I also have the print out from the test they done BEFORE the lamber sensor was changed...and it seemed better before it was changed!! ???

Fast idle test
Engine speed - 2542rpm PASS
CO level - 0.47% FAIL
HC level - 82ppm PASS
Lamdba - 0.99 PASS

Scond Fast idle test
Engine speed - 2608rpm PASS
CO level - 0.82% FAIL
HC level - 246ppm FAIL
Lamdba - 0.97 PASS

Natural Idle test
Engine speed - 872rpm PASS
CO level - 0.22% PASS

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Sun 6 Feb 2011 22:08
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - unthrottled
No Nox? Oh well.

I don't think your catalytic converter is catalysing! See if they can source a second hand cat for you-a new one is quite pricey.

Looking at your Fast idle test (from the pre lambda sensor change) there is 0.47% CO level with lambda=0.99. Well, at a lambda=0.99 you would expect ~0.5% CO in the engine out emissions. You would also expect ~0.5% oxygen in the exhaust (due to dissociation of CO2). if the cat was working, The oxygen in the exhaust should have oxidised the CO to CO2-and the engine should have passed.

NOx levels would confirm this. At any lambda<1.0 NOx levels post catalyser (ie tailpipe emissions) should be low (ie a pass).
Did the garage check the voltage on the lambda sensor before deciding it was faulty?
The lambda sensor measures the oxygen content in the exhaust and the output voltage correlates to this content.
The garage should have measured the exhaust oxygen content and tested the output of the lambda sensor to see if the sensor was functioning correctly.
If not, it looks like they decided that the fault either lay with the cat or the lambda sensor and decided to try the lambda sensor because it was cheaper.
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Mon 7 Feb 2011 08:16
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - nickphonics18
i dont know if they checked the voltage on the lamber sensor.

they rung us and said it could be the lamber or the cat. they were 80% sure the lamber would fix it. so they went ahead with that option.

they come back saying it didnt work. and they took it to another garage for a second reading. they couldnt pass it either.

so now we have it booked in a VW to do a diagnostics test costing another £80 odd!

If the garage has wrongly put a new lamber on should we have been charged for it?

how much do you think a second hand cat would roughly cost?

thanks
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Mon 7 Feb 2011 13:18
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - unthrottled

I thought you might ask about the lambda sensor. As far as I'm concerned, it is possible to distinguish unambiguously between a faulty lambda sensor and a faulty cat.

The job of the lambda sensor is to fine tune the mixture around lambda =1.0 so the cat can catalyse all three regulated pollutants. But even if the cat gets the 'wrong mixture' it is always going to do something right if it is working.

I don't understand why the garage hasn't given you all the figures. Nox is a regulated pollutant so it must be measured. If the garage replaces the cat and this solves the problem, you shouldn't really be charged for the lambda sensor. Whether you pursue it or not, will depend on what sort of relationship you have with the garage. Technically, they phoned you and you made the decision-but I still think the dilemma was unnecessary. They goofed on the diagnosis and they should bear the cost of 'diagnosis by substitution'.

A second hand cat is probably about £80 and a new one £120-150.

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Mon 7 Feb 2011 13:38
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - nickphonics18
the lamber sensor is already fitted now and paid for, but im annoyed that a garage that does MOTs day in day out should understand the emission figures to work out the issues without 'guessing!'

is it possible to test the cat? im going to get VW tomorrow to test the cat before they do this diagnostic test.

all i have is the print out from the emissions test and there nothing on there mentioning Nox figures.

thanks for your help!
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Mon 7 Feb 2011 14:56
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - unthrottled

To be fair to the garage, NOx measurement is not a requirement for the MOT. My garage uses a gas analyser which is capable of measuring NOx and I use the figures from this print out.

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Tue 8 Feb 2011 15:49
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - nickphonics18
VW fixed it so should pass the MOT now!

This is what they said they done ......

problem was something to do with the ecu and they reset something. A sporadic fault with the lambda probe which they had to reset.

what does that mean exactly? so is it an error from MOT garage fitting the lambda?
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Tue 8 Feb 2011 16:01
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - unthrottled

I don't know. I hope they're right-but the lamba ratio looks close enough to 1.0 to get a pass- if the cat works. Get a half price retest, and hope for the best!

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Tue 8 Feb 2011 16:32
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - nickphonics18

VW done a emissions test and we have a passed reading sheet.

so all got do is take it back to MOT place to get the free retest within 7 days :)

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Tue 8 Feb 2011 16:41
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - unthrottled

fantastic! Well, I've learned something today. Thanks for the final update-so many threads are missing 'the last page'...

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Tue 8 Feb 2011 16:53
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - nickphonics18

thanks for your help :)

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Thu 24 Feb 2011 08:02
1999 T reg VW Polo 1.4 petrol - 1999 VW Polo 1.4 - MOT fail on emissions - nickphonics18
how do you reset the service counter on 1999 VW Polo? it flashed up yesterday, obviously the garage didnt reset it

thanks
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