MOT emissions failure on C02

21 Nov 2014
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United Kingdom
I recently got my beloved VW Mk3 Golf MOT'd, I was considering scrapping it and buying something else but we've got a lot of history togetherso wanted to keep her going.

Before The MOT I got 2 new tyres, tracking sorted and a whole new exhaust system because the Cat had gone (except manifold) fitted at Wilko Direct. (The cost of the down pipe and cat was only £100 including fitting, I've been told this is very cheap for a CAT, even for a non type approved one and may be the problem?)

I though she would pass easy now, it did on everything except the emissions!

It was passing the HC but failing the C02 giving reading of around 0.600% out of 0.200% and only just failing on the lambda.

Long story short, I've fitted a new lambda sensor, new air filter, new spark plugs, Forte fuel additive thrashed the car about for an hour to clear out the engine and warm her up and still failing on C02 (but only just passing the lambda now, still passing HC)

I went back to Wilko and they checked the exhaust for leaks and took the car to a garage that they use for another emissions test. Same problem again.
They ordered in a new CAT and fitted it incase the old one was faulty, still failing on C02!!!

I've been to a couple of independent garages and they've basically said that I should scrap it, although it's not worth any money I love the car and seems such a shame to scrap her when it runs absolutely fine and passed everything but the C02 emissions...

N.B. the exhaust is emitting a fair amount of white smoke but not at start up, only when the car has been running for a while and warmed up. I've got no loss of oil, no loss of coolant water and no gammy mayonnaise on either caps. I've cleaned the throttle body and the engine seems to be running fine...

It's done 107k miles so is getting on but was hoping for at least another year on the road...

Any help would be great!!! Thanks
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It won't be CO2 that it's failing on, but CO (Carbon MONoxide). That suggests a rich mixture. Do you have the failed Lambda readings and the HC reading? Also, did it pass at fast idle but fail at idle? I did once, in desperation, drill a few small holes in an inlet manifold to introduce an air leak to weaken off a car that I couldn't gt lean enough. That's a bit extreme though. You could maybe check the coolant temperature sensor. If it thinks the engine is cold, it will be giving it extra fuel. Also check the fuel pressure in case the regulator is jammed and the pressure is too high.
Definitely wouldn't even consider scrapping it, but if you hear that type of talk you just picked the wrong place to get help with repairing it.
Think some of those had problems with excessive back pressure from the sump. So consider testing with the sump vent pipe disconnected from the manifold, see if that changes anything. You will have to block the hole where the pipe goes onto the manifold to get it running properly. Try to get a mechanic sympathetic to the cause with an emission tester, maybe a small local garage?

They were a funny Golf those MK3's a bit strange with the big blocky lights in the bumpers. Just a bit odd ball, but that's probably what you like about it? Some people even like County and Western music, apparently?

Like minded people
Its an inexpensive job to change the CTS first but my experience with cheap non type approved cats is that they are rubbish.
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The car passed the natural idle speed but failed at fast idle. The readings are, at fast idle...

CO - 0.546% Fail
HC - 66ppm Pass
Lambda - 1.031 - Fail

I've since had other tests done and the car was passing on Lambda, but only just, so I think it's mainly a CO problem...

I got a little VAG diagnostics reader off of ebay and it came up with a CO sensor - low voltage code. I erased it, disconnected the battery for a few hours, re connected and the fault had come back. I erased it again, disconnected the battery over night and had a fiddle with the connection of the lambda and made sure it was all plugged in correctly etc and the fault hasn't come back. Although the car is still running the same with plenty of white smoke.

I'm really hoping it will be something small but if it's the head gasket on it's way out or something, I'll give it a bash myself. Just want to diagnose the fault first, if at all possible

I'm somewhat inexperienced with technical engine parts and terminology but am learning a lot fast with this car haha!

Thanks for the help guys! :mrgreen:
Not going to help the OP, but I had my Peugeot 406 done last week and I was surprised at how good the emissions were:-

CO 0.11 ( 0.20 )

HC 53 ( 200 )

Lambda 0.990 ( 1.030 )

Particularly as the exhaust to manifold joint is blowing noisily on the over run AND there is a hole about 7 mm diameter about 100 mm after the CAT !

The lambda reading surprises me a bit. Normally, high CO means a rich mixture. If that were the case, your Lambda value should have been low - certainly less than 1. Is it a cheap aftermarket lambda sensor? They can be troublesome. A genuine Bosch one would be my choice:

If your lambda sensor isn't sending duff information, then the engine isn't running rich and I'd suspect the cat. Any chance of getting a better cat off a breaker?
I don't get involved with breakers any more now that they don't let you go round their stock on your own.

But there is an active recycling market for old CATs in bulk and I would have expected most breakers to send them all off. It seems they can get £30 to £40 each.

In the meantime CATs on Ebay seem a lot less than the £300 manufacturer's prices although one wonders about the quality and performance.

Tony I was quoted £800+ for a genuine Peugeot 406 cat not a couple of months ago! Oh and at that time there was none in the country, or in the factory, according to local Peugeot dealers.
I have just bought an Ebay one for £45 delivered!

Although having said that, what I mean is that I have paid £45 and waiting for it to be delivered!

Next problem will be the impossible to access screws holding it onto the manifold.

Even if I do access it then it will probably snap off.
I'm in a quandary about after market cats.....I've had two that were no use.
One was on a 206 and the other on a C3.......curiously enough, similar engines!
Neither passed the emissions, even when red hot.
John :)
How many do you fit a year?

Two out of two or two out of two hundred?

Who was the supplier?

What maker's name on them if any?

In my case as I have just got the MOT then it will be a year's driving if nothing more!

Two petrols and one diesel this year, Tony......I don't know who made the cats but it was a Walkers exhaust. All was supplied by my local factor, and the diesel cat had corroded away.
No cats the year before.
I'm not saying that aftermarket cats aren't the way to go, but for example the 206 that came in (1.1) showed up with exhaust problems, so on went a complete system (the cat on those bolts directly to the exhaust manifold), two lambda sensors and all gaskets and clamps.
Back to the mot station and it was exactly the same as before :eek:
Replaced the cat with a genuine one and all was well - except I was left with a used cat!
John :)
The Vehicle Certification Agency are often on the warpath with aftermarket cat manufacturers. They're supposed to comply with EU regulations but they know the MOT gas analysers can't measure oxides of nitrogen so they don't bother putting some of the more expensive chemicals into the cat. Knowing that, it wouldn't surprise me at all if they skimped on the other chemicals too!
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