Help - failed MOT on emissions plus error codes on computer

[Adam32]

Despite putting some cleaner though the vehicle, my 2001 Saxo Furio has just failed the MOT on emissions for the Lambda reading on 2nd fast idle. The reading was 1.105 when the maximum limit is 1.030. On the first fast idle test the CO was also high at 0.309 when the maximum limit is 0.300. The MOT tester suggested it could be a faulty Lambda sensor - what do people think?

One other thing to note is I sometimes get the engine management light come on the dash and the car seems to stutter badly. This is an intermittent problem as the last few weeks everything has been okay. When the problem occurred before I borrowed a neighbours computer and got the following two errors:

P0420 - Catalyst system efficiency below threshold bank 1
P0300 - Random / Multiple Cylinder Misfire detected

Do you think these problems are related to the emissions fail (although no lights were on the dash and the car was not stuttering during MOT test.)

Where should I go from here, do you think the problem is a faulty Lambda sensor? Where should I start? The problem is money is tight at the moment, so if I can't fix this myself and get it through the MOT I will have to scrap the vehicle.

[stevo67]

First fault code is the cat,the second is the coil pack which can make the car overfuel.

[Adam32]

First fault code is the cat,the second is the coil pack which can make the car overfuel.


So would you say replace the coil pack (about £25) As for the Cat should I replace the Lambda sensor (first one) Or do I need to replace the one on the exhaust as well?

[Mickswan]

Replace the coil pack first and see how she runs. Can you currently hear a misfire?

[Adam32]

There is no current misfire that I can hear and she seems to be running very sweet. Although I could fire her up tomorrow it it may be bad, there is no telling when it may happen.

The problem is I don't have an emissions tester, so won't know what will work for certain. I have to take the car back for MOT within 10 days for a free retest and if it fails again I will have to pay for another test, so really want to get this sorted.

[Chipwizards]

Lambda 1.1 is lean. That is assuming the sample probe was far enough up the exhaust not to have been diluted by air being drawn up the tailpipe by exhaust pulses.

If it's hot enough the lambda sensor should have the engine running close to lambda 1.0.
Even if the cat isn't working it will still show lambda 1.0 (or very close) if the top lambda is working. The bottom lambda only monitors cat efficiency.
If you have O2, as well as CO present in the exhaust (or values more than the limit) and the cat should be sufficiently hot then the cat efficiency is low. Sometimes a good thrashing up a hill on the motorway can generate enough heat in the cat for it to improve its efficiency.
Short trips are bad news for cats.

[Chipwizards]

If the emission test shows lambda higher than the limit again, simply 'mask' the tailpipe with a rag so that the gasses have to push past the rag and see what happens to the readings. If they stay high, the lambda is bad. If they go to lamnda 1.0, or close to, the test procedure is bad. (sample probe not far enough up the tailpipe).

[Adam32]

Lambda 1.1 is lean. That is assuming the sample probe was far enough up the exhaust not to have been diluted by air being drawn up the tailpipe by exhaust pulses.

If it's hot enough the lambda sensor should have the engine running close to lambda 1.0.
Even if the cat isn't working it will still show lambda 1.0 (or very close) if the top lambda is working. The bottom lambda only monitors cat efficiency.
If you have O2, as well as CO present in the exhaust (or values more than the limit) and the cat should be sufficiently hot then the cat efficiency is low. Sometimes a good thrashing up a hill on the motorway can generate enough heat in the cat for it to improve its efficiency.
Short trips are bad news for cats.

Just a couple of things:

1) Are you saying the bottom sensor would have no effect on the MOT readings? Should I only replace the top then?

2) For information, on second fast idle CO was 0.137 and HC 19

2) You talk about the importance of the cat being hot. What about taking a blow torch to the cat and making it cherry red just before test?

3) Thanks for the other suggestions about the rag, but I only get one free retest and if it fails I have to pay for another test. This would make it cheaper just to buy a Lambda sensor even if it does not need replacing.

[stevo67]

It's the top lambda sensor that is the important one,don't talk daft about a blow torch.Just take the car for a good run before the test and present the car to the tester with the engine running.

[Chipwizards]

above +1

It's the cat's internal temp that needs to be up.

0.137 and HC of 19 ppm look like it's not far off, did it only fail on high lambda then, with those second fast idle readings?

[Adam32]

Yes it only failed on high Lambda.

Anyone know if this is the right sensor for a Furio 2001, the ebay seller said it was, but they seem a bit indecisive.

http://www.ebay.co.uk/itm/CITROEN-XANTIA-1998-01-1-8-16V-FRONT-4-WIRE-DIRECT-LAMBDA-OXYGEN-SENSOR-OS08304-/171658085297

[Chipwizards]

If you connect a diagnostic scanner and look at live data you should be able to see the lambda cycling rich/lean. This will show if the lambda is working or not.

When a lambda fails the engine will almost universally go to a rich condition (lambda<1), which does make me suspect that the measurement wasn't accurate, or you may also have a small leak in your exhaust system which will make the mixture appear to be lean as air is drawn into the system.

[Adam32]

Car failed the Lambda again, so looks like I will be scrapping if anyone wants to make me an offer. Everything else passed the MOT apart from the Lambda reading. I really don't know what else I can do. Things I have done:

1) fit new Oxygen sensor
2) fit new spark plugs
3) new fuel filter
4) New air filter
5) oil change and filter
6) Taken injectors out and soaked them in in injector cleaner.
7) Fit new Coil

I can't keep throwing money at this :wall:

[Chipwizards]

There's absolutely no need to throw any money at it.

Did you do as I said in my last post?

Emissions are very simple.

The mixture ratio needs to be 'stoichiometric', which basically means the 'correct ratio of air and fuel' in order for the cat to do its job.

Correct is Lambda 1.0. Your lambda sensor should see to this if everything is as it should be. You can see what the lambda is doing with a scanner that gives live data readings. If you don't have a scanner you can also see it with a multi-meter with the positive wire to the black wire on the lambda and the negative on the grey lambda wire. The voltage should go up and down from below 0.44 v to above 0.44 v. Typically you'll see it go from 0.15 up to 0.85, but all it MUST do in fact is vary above and below 0.44v. If it does this the lambda sensor, and also the mixture ratio is OK.

If it is OK and your car then fails on lambda IT IS THE MEASURING PROCEDURE THAT IS WRONG.

If it fails on CO, or HC but passes on lambda then it would be your catalist at fault.

Post your actual test results. CO%, HC ppm, Lambda and ideally O2% as well and I will be able to see exactly what is happening.

Has your exhaust got a big silly fat tailpipe?

[Adam32]

Thanks for all your help - all fixed now and have the MOT for another year :y:. I ended up replacing CAT, but then noticed centre pipe ending was deteriorated so had to replace that as well. Plus the back box was badly deteriorated so did that as well. So I ended with an entire new exhaust. Plus all a bit of a nightmare as GSF / Euro kept selling me wrong exhaust pieces!

Before I changed the exhaust I replaced:

1) temp sensor - read this could be the culprit
2) Coil pack (there was a misfire)
3) oxygen sensors top and bottom (could not get the bottom one out the old cat)
4) spark plugs

Other things I did:
A) oil change
B) air filter change
C) took injectors out and cleaned them
D) Engine flush

After all of the above it flew though the emissions tests.


Think I have spent about £350 - so probably more than cars worth:wall:

[Manu]

or you would have binned the car, spent the money elsewhere with added tax and insurance costs. Now you have a new exhaust, new sensors, new coil pack and service is done. How much is peace of mind...