Failed emissions - nightmare?

[rix]

So if a car fails its MOT on emissions how likely is an easy resolution?

I've 'heard' that it may be a nightmare to diagnose and may be an indicator of something serious. Would you buy a car which had previously failed on emissions?

[Slow]

My car will fail on emissions due to a straight through exhaust, need to put a cat on it each time the mot comes round.

But if it fails with a cat it might just need a new one.

[Strawman]

Depends a lot on the car and what it's service history is like, sometimes it can mean an engine rebuild is needed so the car is scrapped, sometimes it just needs a new air filter and an "Italian" tune-up.

[ant427]

my vectra always struggled with emissions,the engine had to be super hot to pass,even with new cat and lam sensor. Luckily the mot guy was good enough to rev the nuts out of it for ages and not just hook it up,test it and fail it.injector cleaner and a good blast might help.

[Slow]

anonymous said:

[redacted]

Its declared to having a "racing exhaust".

The other thing is when my car was made it didnt need or have a cat.

So to put a cat on it is a modification in its self.

[Futuramic]

Slow said:

anonymous said:

[redacted]

Its declared to having a "racing exhaust".

The other thing is when my car was made it didnt need or have a cat.

So to put a cat on it is a modification in its self.

This doesn't make sense at all! I'm assuming that the car is a Datsun Cherry Turbo, as stated in your profile. If so it'll be what '81-84 vintage. It's highly unlikely to have had a cat from the factory and will be eligible for a non-cat MOT test. If a car is built without a cat, a kit car for example, it's tested as a non-cat.

[Slow]

Futuramic said:

Slow said:

anonymous said:

[redacted]

Its declared to having a "racing exhaust".

The other thing is when my car was made it didnt need or have a cat.

So to put a cat on it is a modification in its self.

This doesn't make sense at all! I'm assuming that the car is a Datsun Cherry Turbo, as stated in your profile. If so it'll be what '81-84 vintage. It's highly unlikely to have had a cat from the factory and will be eligible for a non-cat MOT test. If a car is built without a cat, a kit car for example, it's tested as a non-cat.

Not MOTed it myself yet but the previous owner said he had to put a cat on each year for the mot.

[Futuramic]

If it's pre August 1992, so J reg or older then you don't have to worry about a cat test. I'm afraid the previous owner was talking rubbish. At least you don't have to worry!

[Greengecko]

anonymous said:

[redacted]

Anyone clarify if it is 'illegal' to put a de-cat pipe in between MOT's. Relevant legislation?

[Slow]

Futuramic said:

If it's pre August 1992, so J reg or older then you don't have to worry about a cat test. I'm afraid the previous owner was talking rubbish. At least you don't have to worry!

Well thats good news, its mot has been up for 2 weeks since i bought it.

Finding the stupid little front brake discs and rear hose has been a pain.

[MG CHRIS]

Futuramic said:

Slow said:

anonymous said:

[redacted]

Its declared to having a "racing exhaust".

The other thing is when my car was made it didnt need or have a cat.

So to put a cat on it is a modification in its self.

This doesn't make sense at all! I'm assuming that the car is a Datsun Cherry Turbo, as stated in your profile. If so it'll be what '81-84 vintage. It's highly unlikely to have had a cat from the factory and will be eligible for a non-cat MOT test. If a car is built without a cat, a kit car for example, it's tested as a non-cat.

Your car doesn't have a cat as standared so for the mot is tested as a non cat test the cut of date was around 92-93. My mx5 import made in 1992 doesn't require a cat test even though it came with one from factory as all jap cars had cats way before us.

As for the kit cars they need a cat test unless the engine number is the matched to the original car so if the donor was a 90s car and the engine number was original to the car, no cat test, anything apart from that needs a cat test.

You can get away with getting through a cat test without a cat using different maps that control the fuel/air mixture. Don't forget a mot is only test for roadworthiness for that day the car could pass one day fail the next with a blown bulb for example.

[davepoth]

If it's got a carburettor, this is really easy. Drive to the MOT place, open the bonnet, and adjust the mixture until it's so weak that it barely runs. give to MOT place, tell them to definitely not drive it anywhere. As soon as you have the car back, re-set the mixture.

[V8RX7]

anonymous said:

[redacted]

You clearly don't move in modified car circles - this is very common with CATS, aftermarket ECUs having different settings to get through the MOT, different lights, No. plates etc

[Simes205]

davepoth said:

If it's got a carburettor, this is really easy. Drive to the MOT place, open the bonnet, and adjust the mixture until it's so weak that it barely runs. give to MOT place, tell them to definitely not drive it anywhere. As soon as you have the car back, re-set the mixture.

Yep or with mine drive to MOT, switch to map 2 (leaned out mot map limited to 3k rpm). Collect car and switch back to map 1 (power map). Emissions passed.


Back to op, air flow meter, dodgy sensors etc can cause poor emissions.
Your garage should be able to diagnose.

Edited by Simes205 on Saturday 6th April 21:16

[rix]

As an update, it's not my car, but one I was due to buy, subject to a fresh MOT - and it failed! Seller is obv going to try to resolve, but a bit apprehensive that it will be put through via the usual tricks and I'll end up having this battle every year. It's a car with plenty of mods, but it has had a MOT since they were fitted.

[Disco_Biscuit]

rix said:

As an update, it's not my car, but one I was due to buy, subject to a fresh MOT - and it failed! Seller is obv going to try to resolve, but a bit apprehensive that it will be put through via the usual tricks and I'll end up having this battle every year. It's a car with plenty of mods, but it has had a MOT since they were fitted.

It sounds like you might have issues next year maybe. If that bothers you walk away, plenty more cars for sale.

[Mr E]

Greengecko said:

Anyone clarify if it is 'illegal' to put a de-cat pipe in between MOT's. Relevant legislation?

I did this every year. All modifications declared to the insurance company.

The Elise has just failed on emissions. Sports cat is in bits (again).

[Strawman]

Greengecko said:

Anyone clarify if it is 'illegal' to put a de-cat pipe in between MOT's. Relevant legislation?

Yes very unlikely you will get stopped for it but a car must be able to pass an MOT to be roadworthy, hence being stopped for non working brakelight and getting a producer to show it has been fixed within three days. I've seen a police check point for emissions at the side of the road in London before, they were pulling over likely transgressors and testing the exhausts. If you failed a random test because of some component failure then you probably just get told to fix it, if you fail because you've removed something like a CAT then harder to explain.
ETA- if a car can pass the emissions limits for the relevant year without a CAT I'm not sure if it is illegal, but could leave you arguing the toss, hence some people bore out the Cat rather than replacing it with a bit of pipe.

Edited by Strawman on Sunday 7th April 10:31

[steveo3002]

rix said:

As an update, it's not my car, but one I was due to buy, subject to a fresh MOT - and it failed! Seller is obv going to try to resolve, but a bit apprehensive that it will be put through via the usual tricks and I'll end up having this battle every year. It's a car with plenty of mods, but it has had a MOT since they were fitted.

could be anything causing it..lack of use , stale fuel , failed sensors , worn out engine , lack of servicing

[V8RX7]

rix said:

So if a car fails its MOT on emissions how likely is an easy resolution?

I've 'heard' that it may be a nightmare to diagnose and may be an indicator of something serious. Would you buy a car which had previously failed on emissions?

It depends what needs fixing.

It's like saying would you buy a car with a puncture - if it's fixed correctly it's fine if it's bodged it could be dangerous.

So I'd wait and see what they say