petrol engine, normally aspirated, failed on CO (over 3x limit, 0.73%) but hc and lambda OK

Mr MOT tells me that's a classic catalyst failure, which is interesting as I had a new exhaust & cat fitted a couple of months ago

Is he right? Is that the only likely cause? (of course I'll be talking to the place that fitted the exhaust but I'd like to have a bit of a clue beforehand)

Ta in advance…

I'm presuming it's petrol if it's failed for CO?

Your catalyst is likely something like this:

So there could be something wrong with it but then why would it not fail for NOx as well? And as you say it is fairly new.

Another potential reason could be something like the fuel/air mixing in the engine. If the ratio is more heavily weighted towards fuel than fuel:O2 ideal then the CO might be symptomatic of incomplete combustion due to relative lack of O2.

I'm not an expert on exhaust and combustion technology, more what comes out of the exhaust, so I don't profess to know everything but those are a couple of reasons that spring to mind

EDIT: just reread that it is petrol!

sometime a dammed good thrashing can help!!
seriously.

Cynical first.

Did the garage really fit a new catalyst two months ago?

Simply the oxides of nitrogen are stripped of the oxygen which then reacts with CO to give carbon dioxide.
sounds as though the catalyst is pooped. do they test for oxides of nitrogen?

For me two main reasons why you'd screw your Cat in just two months would be engine having excessive oil use – Cat then failing to burning to burn the oil and being overcome.. or you've beached it on something offroad, speed bump etc.

Or – exhaust was replaced and they didnt bother with the cat (not cheap things either are they?).

Is there a way you can open it up and check a manufacturer number etc?

Go back to exhaust centre as you have warranty right?

MOT-local garage or rip off chain?

thanks

I know what a cat is supposed to do (broadly) and I know that this one is failing one one of 3 criteria

I just need to know if this is "definite" cat failure or other potential causes, so I can have a meaningful "discussion" with the people who fitted it

another method of killing the cat is running too rich. Engine operating OK at all times? Obviously if it were runnign rich during the test it would be picked up by the 02 sensing.

Question whether cat was replaced, often exhausts are done without the cat being replaced as thsoe sections are often very thick and more resiliant – cheaper to just replace the cat back.

Overfuelling can **** a cat pretty quickly as it contaminates the catalist. Take the car out and give it a dam good spanking and take it back to the garage with the exhaust manifold glowing!
Does the car use much oil or water?

If it's not failing on NOx then it might suggest that the CAT *might* not be a fault. I'm sure they can test these sort of things though? How close is the NOx to the limit? What age is the car?

Genuine cat or patten part from a fast fit? If from fast fit will not be genuine and could easily fail. Have seen them go within a couple of weeks before.

Doesn't use much oil or water – isn't a very smelly runner & not aware of lots of smoke

It had been driven at motorway speeds (or very slightly above 😳 ) for 30 min prior to test as I was runing late

Exhaust was replaced because previous cat had fragmented and moved within housing, causning big running problems so I'm pretty sure they had to do it with a new part

Would an oil-contaminated cat fail on just this criterion ?

Wasn't given a NOx result

CO – 0.73%
Hc – 0.02
Lambda – 1.011

How much did they charge you for the cat and exhaust?

Wasn't aware anyone tested NOx, don't think my MOTs have every included it, though they are on older cars so don't have any limits anyway.

ck ok, I thought they were as vehicles have to meet EURO standards for CO, NOx and PM10 but a bit of googling reveals that it is not a standard for an MOT – bizarre. Conversely HC is not a EURO emission standard if you're car is newer than 2000 but tested in the MOT.

I would speculate that the rationale is that measuring CO and HC is a good enough measure of a 3-way catalyst's performance

So the consensus appears to be that you need to get the catalyst checked to see whether they definitely replaced it or whether it is damaged. If it is damaged or spent, that could be because it is a crap part or something else is happening further up from the catalyst and has damaged it.

If it isn't damaged/spent then your engine may need tuning to provide a better fuel:air(O2) mix as that would be one potential explanation behind high CO levels that even the catalyst couldn't handle – if this ratio is really out then the catalyst might have been spent in such a short period.

your engine may need tuning to provide a better fuel:air(O2) mix as that would be one potential explanation behind high CO levels that even the catalyst couldn't handle – if this ratio is really out then the catalyst might have been spent in such a short period.

His lambda (O2 content) is correct, so the tuning is fine at test time. 🙂

doh, should have read that bit properly!

In that case I shorten my summary to….. get the catalyst checked to confirm replacement!

FWIW, booked in for a warranty replacent next week

Good outcome, if the exhaust place does MOTs as well, it would be worth getting them to put the probe up the exhaust before & after, to see what difference you get.

What car is it on as well? does it have an EGR system as well?

Glad you got it sorted

I will indeed encourage them to shove something up it's backside after changing (not certain they do MOTs there though so maybe not)

E39 beemer, about 14yr old
EGR, doubt it – how would I tell and what would it alter ?

E39

This thread is useless without pics!

I almost bought a friends 10yr old one (owned since new) but she was having electrical problems with the stereo/heating that she had trouble identifying the cause. I just couldnt face driving round in a coat or in silence!

i've a E39 too, my cat failed due to neat petrol, as I'd lost the spark to cylinder. Didn't notice it…, well when you've still 7 working.

It was only the noise from the cat i noticed.

Could it be more likely that you haven't fixed the origianl problem?

Other cat killers:

Puddles. If your cat is underneath the car, rather than up off the manifold, driving through water can crack the cat internals.

Bump starting. Can throw neat fuel in to the cat – don't do it.

What age is this car?

Similar happened to me, was down to not using a genuine cat after fitting a patern one 3x