('03-'05) Catless uppipe, sports cat downpipe, what to do to the ECU. - Subaru Forester Owners Forum
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post #1 of 2 (permalink) Old 04-12-2019, 03:33 PM Thread Starter
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Catless uppipe, sports cat downpipe, what to do to the ECU.

Hi I’m going to be putting an STi catless uppipe with a resistor on the o2 cable for now, and a sports cat downpipe with second cat removed. My question is a: will I end up with a check engine light? b: I’m totally novice on tuning but quite technically minded; is there a relatively simple fix that can be done to compensate for what I imagine will end up being quite a lean mix?

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post #2 of 2 (permalink) Old 04-15-2019, 04:53 AM
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I've fitted an up pipe decat as you propose and after fitting the resistor, I had no problems and did not need a remap/tune.
I've also fitted a catback exhaust (so no other cats were touched) and did not require a remap/tune.

I am still on the factory map and been on a rolling road to make sure the fuelling etc was ok - All was good.

If you want to start changing cats in the downpipe or under the floor, you will almost certainly need a remap to be safe. If you run lean you could end up causing piston or valve damage.

I guess in theory, if you knew how the air fuel oxygen sensor responded in terms of resistance to running rich/lean, you could add a resistor in the circuit to fool it to run richer to compensate for a change of downpipe cat...In theory - I don't know anyone who has done that as most simply stay off boost after fitting a sports/decat downpipe etc until you get a remap/tune.

If you fit a decat in the downpipe, you will struggle to pass, or simply flat out fail any emissions testing if that is a concern - Some members on here have reported smells of exhaust gases being very noticeable inside and behind the vehicle after fitting a decat in the downpipe/underfloor and revert back to a sports cat for performance without any noticable exhaust gases

Do you have a particular power goal in mind?

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