MY03 WRX
I have been looking at replacing the downpipe and was talking to a vendor today and he said that if i replace the downpipe with a striaght through piece, i may well get the engine warning light coming on as it can't detect a temperature difference from one side of the cat to the other, but this is cured by soldering a resistor into the ECU?, IS this correct??
cheers
James

 

You will likely get the check engine light and the reason is the missing cat but it has nothing to do with temperature. It's the oxygen content of exhaust gases that will be different and will eventually trigger the P0420 ECU error code (catalyst efficiency below threshold). There are two lambda sensors in WRX exhaust and the ECU is monitoring the differences between the readings of these two.

Soldering a resistor in series with the rear lambda can make the CEL go away but it is not a reliable solution.

There are two ways to avoid the CEL:
- Install a downpipe with a high-flow sports cat
- Have your ECU reprogrammed to ignore the rear lambda

Note that the ECU reprogramming is the only 100% sure way of not getting a CEL as a result of downpipe change. Some sports cats are apparently not efficient enough.

I have a Hayward & Scott sports cat downpipe and haven't gotten a CEL yet.

 

Thanks for that ... i meant to say oxygen not temperature - couldn't remember clearly enough from this morning what the guy said!! LOL

So is the ECU reprogramming bit easy enough, can it go to a main dealer to do that that or would a specialist be best?

I have been looking at high flow cats, but hey are so expensive at around 475+VAT!!!!!
IS there anyting cheaper than that?
James

 

As far as ECU reprogramming for ignoring cats, main dealers don't do these kind of things. You need to find an EcuTek tuner. As a bonus you will get your car remapped for more power. But it ain't cheap either, I think you are looking at very similar price than what you would pay for that sports cat downpipe.

The resistor trick with the rear lambda does work for some people. It's just that you may be the unlucky one for who it doesn't. I think Scoobysport sells a somewhat more advanced lambda-fooling circuitry that works better than just a resistor but I've heard even that doesn't completely kill the CEL in some cars.

The Americans at NASIOC have also devised a mechanical fix that involves relocating the rear lambda slightly away from the exhaust flow.

So as you can see there are several possible ways to try to fool the ECU into thinking the cats are still in place but none apart from total reprogramming is completely reliable.

I went for the sports cat because the car stays emissions legal and I felt the peace of mind and no parts swapping at MOT time was worth the extra price.

 

Yep - i've been dropping the cat on and off for years, but seeing as this is the newest modifyable car i'm ever going to have, its probably wise to get the sports flow cat.