Downpipe Turbo Subaru FAQ
Many aftermarket engine management systems can also be used to prevent the CEL light from showing up.
Which is the best CEL fix, mechanical or electrical? Both work fine, though personnel living in states with OBDII testing prefer the mechanical fix. The reason for this is during state mandated OBDII testing, the electrical fix can show up as a malfunctioning sensor. For some states, this one malfunctioning sensor is fine, for others, you will have to fix it and return. In the end, the mechanical fix is cheaper and easier for most users.
Can I drive around with a P0420? Yes. This is not a code that will throw your car into limp mode, so you can drive normally with it until you get a CEL fix. It’s never wise to do so though as a CEL is easily seen by police who may then do a “safety inspection” as well as cause issues during any local emissions testing.
Do I need engine management with a downpipe? For any engine modification it’s extremely wise to have your car tuned. For 04+ WRX users, it’s pretty much a requirement as the open loop/closed loop fueling issue mandated by the EPA causes long term reliability problems with using a downpipe and no tuning. For STI users, the STI was the one exception manufacturers are allowed to have, but it’s a wise move regardless as the STI and 06+ WRX users have a good chance of boost creep occurring with downpipe use.
Should I reset my ECU after this mod? It is never a bad idea to perform an ECU reset after a mod. The traditional route is to disconnect the negative battery terminal, press the brake pedal for a few seconds to bleed the system of charge, and reattach.
What about emissions problems like emissions tests and legality? Using a catless downpipe is a violation of section 203 of the Clean Air Act. Each state has statutes that cover this subject as well. As far as passing state emissions tests, as long as you still have one cat in your system, you should be fine unless you run into a sharp-eyed and knowledgeable inspector who knows there should be two in the main exhaust. The exhaust should be properly warmed prior to any testing to ensure it is operating efficiently. Another smart move is to find someone in your local area with the same set up to compare notes.