Hoping someone out there can help me, as i am having conflicting advice from mechanics. I failed my MOT today on CO emissions, fast idle i was 1.28%, 2nd fast idle 1.3% (both should be below 0.3%) and on natural idle i was 2.29% compared to 0.5% allowable. Now i have a 99T plate WRX, and have recently had a scoobysport decat centre section and backbox fitted. They are adamant that emissions should still pass based on the front cat doing most of the work. A diagnostics expert i phoned suggests i refit a centre cat and then take things from there but i dont want to have to buy one of these if it doesnt solve the problem! I have read on here about fluctuating idling revs, mine has that too recently, are these connected?

 

The cat in the DP should be enough to pass the MOT. Given your set-up, it should have got through - so there's something wrong, either with the cat in the DP or with the lambda sensor (these are the most likely - could also be the MAF but shouldn't be because of the emissions you are getting, AFAIK).
Out of these parts, the most likely thing to go wrong is the MAF, then the lambda, then the DP.
Fitting the centre may get round the problem, but if it's the MAF of the lambda that's gone wrong you want those fixed; if the cat in the DP isn't working (unlikely), the centre cat could do the job. But again, you'll then want to either get a new decat dp, either with or without the cat.
As for finding out which one's actually wrong - can't help you there much, I'm afraid!

 

Thanks for your reply. Looks like i shall be paying Falkland performance a visit to get the lambda looked at!

 

on that age of car, maf was much hardier.

much more likely to be lambda.

Also front cat will do bugger all unless its hot. should ahvetaken it for a blast before hand.

Finally, have a look at V5, if the car is a 2 axle rigid body and not declared as an impreza, then car should not be expected to meet emissions tests for impreza and pre (cant remember the year) regulations ie. before cats should be applied.

Get the MOT station to check the rule book.

 

I had the same problem , but what i did to get past the mot was i took the airfilter out and gave the car a good running and took it back...it passed..hehe

 

Hi guys, well i have looked at the V5 and it is declared as a 2-axle-rigid body under the model/type entry, it is mentioned as an impreza under the make category. I will give that a try though and phone them.

 

Done a few checks online, seems the catalytic converter thing came in on K reg cars, 1992.

 

Mine did the same last week (93wrx same exhaust set up) failed first test despite being "warmed up" The mechanic then retried test using the lower rev limits for each test (ie2500rpm as opposed to 3500 rpm) and it flew through. My car also is a bit irratic at idle. Still don't know why it did it but I've got my MOT so until next year...

 

What are the limits for a P1 coz the're based on an STI5 aren't they, with only 1 cat I believe. I noticed that the mot computer listed "turbo 2000" and "P1" so maybe it could be tested as one of those instead???

 

Not sure, the printout i have just shows subaru impreza, with the cc written in pen. Whether that indicates they havent tested an impreza before i dont know but i guess its possible.