Renault 0.9 TCE failed MOT on emissions - Lambda & CO

[Alex Z]

Our Twingo failed its MOT today on the emissions test, with CO and Lambda too high on the first test, but only Lambda on the second which seems a little odd.

For context, it's 9 years old and has done less than 20k with plenty of short journeys, but has been regularly serviced.
Last summer it had the camshaft position sensor swapped as it was logging fault codes and getting tricky to start. Since then it's been better but not quite as quick to start as it should be, though there's no codes being logged when I last checked.

Any suggestions on likely cause? Air leak? Faulty MAF or Lambda sensor? Dirty engine from short journeys?

Either way, it'll be going to a garage for investigation but it would be nice to have a clue first.

Fast Idle Test
CO 0.28 (limit 0.2)
Lambda 1.173 (limit 1.03)

Second Fast Idle Test
CO 0.07
Lambda 1.180


Annoyingly it also failed on the windscreen washer jet not working, which I tested earlier today. One of those rare cases where it genuinely stopped working on the way to the test.

[E-bmw]

Don't call it a lambda sensor call it an oxygen sensor, and then the answer is really easy.

There is too much oxygen in the exhaust, so there will be a leak in the exhaust.

[stevemcs]

These can split the back box, the last one we had failed on the same reading. Pretty sure the exhaust is cheaper from merc than it is Renault although it’s still stupid expensive- something like £700.

[Alex Z]

Thanks both.
Sounds like the exhaust is a likely culprit for the high lambda reading, but wouldn’t high CO indicate unburnt fuel from too little air? Two separate faults?

[TwinKam]

CAT wasn't properly hot on first test, that's why you get a second shot, which it passed.

[E-bmw]

TwinKam said:

CAT wasn't properly hot on first test, that's why you get a second shot, which it passed.

^^^ Wot 'e said.

[Alex Z]

I spoke to a couple of garages today though didn’t have time to give it much thought.

One recommended a tank full of super unleaded a bottle of fuel additive cleaner and a long drive to see if that does the trick before spending any more money.

The other suggested that a lambda sensor replacement would be the best place to start.

[stevieturbo]

Alex Z said:

I spoke to a couple of garages today though didn’t have time to give it much thought.

One recommended a tank full of super unleaded a bottle of fuel additive cleaner and a long drive to see if that does the trick before spending any more money.

The other suggested that a lambda sensor replacement would be the best place to start.

waste of money, waste of money

It's another screaming of an air leak with that very high lambda reading.

[Alex Z]

stevieturbo said:

Alex Z said:

I spoke to a couple of garages today though didn’t have time to give it much thought.

One recommended a tank full of super unleaded a bottle of fuel additive cleaner and a long drive to see if that does the trick before spending any more money.

The other suggested that a lambda sensor replacement would be the best place to start.

waste of money, waste of money

It's another screaming of an air leak with that very high lambda reading.

I wasn’t convinced by either opinion, certainly.

[E-bmw]

Alex Z said:

stevieturbo said:

Alex Z said:

I spoke to a couple of garages today though didn’t have time to give it much thought.

One recommended a tank full of super unleaded a bottle of fuel additive cleaner and a long drive to see if that does the trick before spending any more money.

The other suggested that a lambda sensor replacement would be the best place to start.

waste of money, waste of money

It's another screaming of an air leak with that very high lambda reading.

I wasn’t convinced by either opinion, certainly.

The first answer to your question above IS the answer, you didn't need to ask any garages, they just want you to pay them to do things/try again.

However as you have asked 2 garages & received complete bullsh!t answers, at least you know NEVER to take a car to them to fix.

[Alex Z]

E-bmw said:

The first answer to your question above IS the answer, you didn't need to ask any garages, they just want you to pay them to do things/try again.

However as you have asked 2 garages & received complete bullsh!t answers, at least you know NEVER to take a car to them to fix.

Yep, that was the intention. Ask the question and see if they seem to have a clue.

It went somewhere else yesterday where they put it up on the ramps and could tell straight away that there was poor back pressure from the exhaust, almost certainly from a leak at the flexible joint. It’s booked in for them to sort it next week.

[jock mcsporran]

We had a similar recent MoT failure but on a Clio. Exhaust was leaking just in front of the back box. You could feel it when you put your hand around the weld.
Solution was new exhaust and it sailed through afterwards.

[Alex Z]

It’s the first real problem we’ve had with it in nine years which isn’t bad. Not the stereotypical French reliability

[GreenV8S]

Alex Z said:

poor back pressure from the exhaust

What do you think that means?