Matt_STi

I got a fault code of P0420 which is flagged up at the cat/lambda sensor. This is for a 2005 STi

I've already replaced the lambda sensor so it can only be the cat.

The only place that I can find that stocks them is either Eurocarparts £240 + VAT or ebay £150 delivered. Both of these in the picture appear to quite large sections of exhausts. Correct me if I'm wrong but isn't this the cat? If so, where is a good place to get a cat from? Not the kind that meow either.



If that is the cat, then what section is this? or do both sections consist of the cat convertor?



Thanks

Matt_STi

No one?

bludgod

iTrader: (3)

did you replace the front o2 (in the headers behind the drivers front wheel) or the rear O2 (the one you have pictured there with the yellow cable)

P0420 is indeed catalyst efficiency below threshold but as the car doesn't have a sensor inside the cat the only way it can know this is from the rear o2 sensor reading.

The rear o2 is in the mid-pipe which is after the cat (cat is in the downpipe - the big fat bit in the pipe after the turbo)

your first pic there looks like a prodrive/sports cat in there I wouldn't think it to be at fault as they are good for lots of ponies (unless your car is modded up to the hilt or has recently has some kind of other failure)


This is a pic of a WRX exhaust but it's very similar. Your STI doesn't have the pre cat, the "front cat" is the the first pic you have there (fat bit after the turbo) and you have the PPP midpipe with no rear cat in it (where the yellow o2 is plugged in).


I would say pickup a second hand rear o2 off someone like mattybr5 (he was having a sale recently I think?) and if it's still giving you grief you can pickup another prodrive cat in the for sale section here easily enough.

Matt_STi

Ah OK, I do remember now after you saying, it is a sports cat. I have already purchased a second hand second sensor (as shown in picture) which didn't help so I think it must be the cat. Don't suppose you know if there is one in the for sale? If not, do I just need a new sports cat? Its been mapped to this so I presume it'll be a sports cat replacement?

gazzawrx

iTrader: (11)

Try giving the car a good run down the motorway, ideally a high speed run i.e 100mph+ if situation allows and see if that helps. It may be the cats clogged up with crap.

Matt_STi

I've tried that previously, wasn't any help. I found a replacement on eBay for £135 so might give that a go.

Matt_STi

So if I replace the Prodrive sports cat for another sports cat, will I require a remap or should it be ok?

forgedmarco

iTrader: (10)

Should be fine just get it checked over by the mapper after

Matt_STi

Thank you. Will get a new sports cat then

STI8Dave

Are you sure that the rear o2 doesn't just need deactivating by your mapper?

Matt_STi

When I brought the car the engine management light wasnt on for around 3 months and it was mapped before I purchased it so I presume something has gone. I'm also getting a whiff of petrol inside the car.

STI8Dave

Mine also stayed off for a couple of months and then one day it came on. Everytime I reset it, it would come straight back on until it was mapped out. I'm not saying its defo the same prob, but sounds very similar.

Matt_STi

Hmm OK, I think I'll go for the new sports cat to see, if that fails I'll get it mapped out. I presume that doesn't cost as much as a full remap?

STI8Dave

The guy I use charges £25-50 for tweaks like that. Be sure to let us know when you sort it

Matt_STi

Will do, going to try and get a Japspeed sports cat first. thanks for help everyone.

Matt_STi

quick question before I splash the cash on a new cat / sensor. Could the fault code P0420 be coming up because of the sports cat? I don't want to buy a new sports cat if the light will come back on again if it just hasn't been mapped to have a sports cat?

Would the most sensible and cheapest option be to have it mapped so the light is off?

bludgod

iTrader: (3)

you can have the code turned off in the ECU - but it won't change the smell of petrol you have which could be a different fault.

If the car is over fuelling for some reason then it might appear to the ECU that the cat has failed - even though it hasn't.

Have you done the basics, had a look round for anywhere fuel might be escaping from or any exhaust leaks anywhere?

Matt_STi

Yeah I've checked that, I've spoken to a friend who said get the front o2 sensor changed and considering its the cheapest option I'm going to give that a go first. Plus it's quite easy to change over so might even attempt it myself!

bludgod

iTrader: (3)

if you can jack a car up and have a big *** 22mm spanner then you should have no issues switching the front O2 out. If it doesn't want to come lose one of the wee blow lamps you can pickup in halfords/camping stores shoudl help you out. That or jack the car up the night before and soak it with penetrating oil/wd40 every few hours, next morning it should slip straight out.

i got my new sensor from mattybr5 on here for a good price and it's working away here like a charm

Matt_STi

I spoke to Mattybr5, he doesn't have any available. I found a place on ebay which is doing them for £45 - comes with a year warranty so least I can see if that clears the issue.

bludgod

iTrader: (3)

if your getting them off ebay make sure they are the right brand. Whilst another sensor brand can also technically be a 4 wire sensor - the calibration range might not be the same.

The ecu has a voltage calibration table that it uses to convert the o2 reading into an AFR/lambda reading - changing the sensor makes the car think it's leaner/richer than it actually is.

Matt_STi

OK thanks for the heads up. the listing says its a OEM replacement but I will check

bludgod

iTrader: (3)

aye so long as it's a denso one you should be all good in the hood. There are pleanty of cars breaking though you could save a lot of cash on a 2nd hand sensor.