Hi all, my S4 TT has just failed MOT on emissions (which have never been a problem before). Before the test I went for a 60 mile blat up the motorway so she was nice and warm, but the CO reading was 3.9 - yes that decimal point is in the right place, the allowable limit is 0.2!

 

Having a search through previous posts I tried disconnecting the two hoses by the balance tube, one at a time, and that dropped the reading to 2.0 but still 10x the limit. Sadly my car is just new enough that it has to have a post-Cat test not Basic Emissions.

 

Just looking for suggestions on what to try next. I’ve got a new air filter arriving tomorrow and have found the instructions on how to check the ECU for lambda sensor error codes. The idle when warmed up is rock solid at about 900 with air con off in current temperatures, and the lambda reading on the MOT was borderline but ok.

 

It has crossed my mind that previous testers may have assumed it had no cat, so the “try another garage” option is already on the table!

 

Thanks for any ideas

 

Chris

What was the HC reading on your emissions print out mate?

Sorry I didn’t get a printout, I was just watching the data on the screen. The CO and Lambda were the ones that were going red at times, the third number (which I guess was HC?) was ok all the time

Sorry I didn’t get a printout, I was just watching the data on the screen. The CO and Lambda were the ones that were going red at times, the third number (which I guess was HC?) was ok all the time

 

You should have been given a printout by the testing station.

 

Its possible that the cat just wasnt hot enough. This is critical on aged catalytic convertors.

 

Seeing the data from the printout is necessary to help diagnose the fault. Thats why we issue the emission report after a test.

What I have found is last years printout which confirms it was a non-cat test! That time the HC was 96ppm, and the CO 0.2. The car was definitely warm this time, I’d just done a long blast. I’ve never needed to do that before, but it was always a non cat test!

What I have found is last years printout which confirms it was a non-cat test! That time the HC was 96ppm, and the CO 0.2. The car was definitely warm this time, I’d just done a long blast. I’ve never needed to do that before, but it was always a non cat test!

Whats the date of registration for your z?

 

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First of Feb 1996

The trying another testing station is not as easy as it used to be as as soon as the second test station log in they will see the fail and why.

 

So its likely the problem is not new as you said previously the test has been done as a non cat test, so, what you need to be looking at is air flow, a ConZult session will help massively, you need to see what the afm is reading, from this you will be able to see if its faulty, or there is an air leak, an air leak will cause all sorts of issues with emissions, also ignition timing and even cambelt timing can and will effect the way the combustion occurs and subsequently the emissions, I would be getting it off to Jim at Powerzed to be checked out or you will end up going around in circles.

 

Jeff

Thanks Jeff. Jim has previously had the car on Conzult several times and it’s been fine so it probably is time for another trip to Avonmouth! Just looking for quick diagnostics in case a round trip wasn’t necessary. You make a good point about the fail being logged, I just checked on DVLA!

The trying another testing station is not as easy as it used to be as as soon as the second test station log in they will see the fail and why.

 

So its likely the problem is not new as you said previously the test has been done as a non cat test, so, what you need to be looking at is air flow, a ConZult session will help massively, you need to see what the afm is reading, from this you will be able to see if its faulty, or there is an air leak, an air leak will cause all sorts of issues with emissions, also ignition timing and even cambelt timing can and will effect the way the combustion occurs and subsequently the emissions, I would be getting it off to Jim at Powerzed to be checked out or you will end up going around in circles.

 

Jeff

FYI when logging on a vehicle for test it doesnt give any indication of it being previously tested (pass or fail) it will only show a previous pass if you attempt to test a vehicle which has more than 1 month left to run.

 

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FYI when logging on a vehicle for test it doesnt give any indication of it being previously tested (pass or fail) it will only show a previous pass if you attempt to test a vehicle which has more than 1 month left to run.

 

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That is interesting, so the system does not show historic testing? that is odd and given the tester could check it on a free app on his phone makes the log in system seem very limited:sad:

 

Jeff

That is interesting, so the system does not show historic testing? that is odd and given the tester could check it on a free app on his phone makes the log in system seem very limited:sad:

 

Jeff

But it serves no purpose to show it has been previously tested to us. If one VTS fails your car you can drive to another and get a pass.

 

If the system automatically showed us it had just failed then testers would be unfairly looking at those previous fail items.

 

Every test must be carried out using the testers own view of the vehicle not someone elses.

 

Only when testing the vehicle at the same VTS will it show as a re test (if it meets criteria for a re test).

 

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So I *might* be able to get through elsewhere if they don’t think too hard about the registration date, but looking at the results from last year’s gas test the CO was 0.2. This year it was 3.9, which would fail even a pre-cat test, until I started pulling hoses. It does seem like something has gone out of whack. Hopefully a duff sensor or blocked filter rather than ruined cats!

So I *might* be able to get through elsewhere if they don’t think too hard about the registration date, but looking at the results from last year’s gas test the CO was 0.2. This year it was 3.9, which would fail even a pre-cat test, until I started pulling hoses. It does seem like something has gone out of whack. Hopefully a duff sensor or blocked filter rather than ruined cats!

Your best option is to find and repair the fault. Another vts may find more problems than you have already.

 

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Good point Chris. I’ll get the new air filter on and check the lambda sensor codes, then get another test done when it’s nice and warm. If (when?) it fails that I’ll be sure to get a printout then get on the phone to Jimmer to book her in!

Good point Chris. I’ll get the new air filter on and check the lambda sensor codes, then get another test done when it’s nice and warm. If (when?) it fails that I’ll be sure to get a printout then get on the phone to Jimmer to book her in!

Temp sensor for ecu could be another culprit. But a diag session will show some needed info for a proper answer.

 

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Thanks for the tipper! Just looked up the correct readings for that, and at least that’s an easy one to reach and test.

Found the answer to getting an MOT without having an emissions test..................take a wheel off, see below from the MOT web site ....Lol

 

MOT

Most vehicles have their exhaust emissions tested as part of the MOT.

 

Usually, they’re tested using a meter. Petrol vehicles used before August 1975 and diesel vehicles used before August 1979 are tested by sight.

 

If your vehicle fails the emission test it fails the MOT. The problem must be fixed before you can get an MOT certificate.

 

The only vehicles that don’t have an emissions test as part of the MOT are:

vehicles with less than 4 wheels

those with 2-stroke engines

hybrid vehicles

quadricycles

 

 

https://www.gov.uk/emissions-testing

Or put a dynamo on it and claim it's a hybrid.:whistling:

Have you not seen the “top secret” new 2017 300ZX Nissan are working on ???

Yep that will do it !!!!!!!!

 

Was there not a pipe that you could take of to cheat the test.

 

http://www.300zx-twinturbo.com/tech/co/co.html

Having a search through previous posts I tried disconnecting the two hoses by the balance tube, one at a time, and that dropped the reading to 2.0 but still 10x the limit.

 

 

 

Was there not a pipe that you could take of to cheat the test.

 

http://www.300zx-twinturbo.com/tech/co/co.html

 

Even if it did work, it's not fixing the problem that the car is running incredibly rich.

Even if it did work, it's not fixing the problem that the car is running incredibly rich.

 

You have a point there Alic :D

Got tomorrow afternoon off to play with my multimeter and see if I can track down the problem. Thanks for all the tips. Well, maybe not the one about the wheels... ;)

OK, I THINK I might have found it! After changing the air filter and checking the TPS and coolant temperature sensor I ran the Mode II diagnostics. The LH lambda sensor reads fine, but the RH one does not - it stayed stuck on Rich most of the time, occasionally flipping to Lean. That would (I think) explain the test results, which had good CO until the lambda sensors kicked in, then the CO went sky high.

 

Obvious answer is to replace the RH sensor and see if that does the trick. Question for the panel though - what happens when a lambda sensor is disconnected? Will it default to lean, rich, or something random? Just wondering if in the short term it would be worth pulling the plug on the RH sensor and getting it tested, or would I be wasting my time?