This is a do it yourself Check Engine Light (a.k.a. CEL) fix for the WRX for use after installing an aftermarket downpipe (a.k.a. DP) with either a high flow catalytic converter or no catalytic converter (a.k.a. Cat) at all and you are getting a cat efficiency code.
If you already know how to do this, or you have seen similar threads, then that's awesome, just don't reply to my thread saying, "There is already info on this out there" or "you just need to do a search and figure it out". I did both of these and figured it out myself. I just don't think that if someone wants to do this exact same mod, they have to search and piece together information from different threads. This is a specific how-to article for this exact mod only... the information has been pre-pieced together for you.
Ok... the idea behind this fix is to basically confuse... well, actually, not confuse... trick is a better word... trick your car. By pulling the O2 sensor out of the direct flow of exhaust and isolating it inside its own little space confuses the car into thinking that the air coming through your new catted or catless DP is clean.
This fix in this thread was performed on a Catted Helix Downpipe on my 2005 WRX sedan.
Ok... so first of all, this is what you will need for this fix.
1. Two 18mm spark plug non-foulers (a.k.a. anti-foulers). I bought the "HELP!" brand (part #42002) at my local pepboys, however they can be found at many auto parts store out there. Cost: $4.50
2. A 1/2 inch drill bit. Cost: $12.50
3. Access to a power drill or drill press
4. A vise... a table vise, bench vise, workshop vise, whatever you want to call it. You just need one.
Alright... here we go.
Step 1. Raise the front of your car up on jacks. Remember: When putting your car up on jacks to always be on a level surface and firmly set your parking brake.
Step 2. If the sensor hole in your exhaust is already unplugged, great! If it's not, Unscrew the plug from the rear sensor opening in the rear section of your new downpipe as seen below. If you didn't remove the oxygen sensor from your car during your DP installation then cut the zip ties that are holding the cable down and unplug it. Once it has been removed from the car you can wipe off all the old exhast dust from the sensor with a dry towel. Set it aside for now.
Step 3. Take one of your spark plug non foulers and drill a 1/2 inch hole through it to make clearance for the O2 sensor to fit inside it. This can be done with a power drill and a vise... the metal is relatively soft. See before and after below:
Step 4. What you are going to do now is screw the oxygen sensor in the non fouler you just drilled and then into the non fouler that you have not drilled... like so:
Step 5. Put this setup into a vise and tighten down both connections with a 22mm wrench.
Step 6. Screw this whole setup into the rear hole of your downpipe. Screw it in there pretty tight as you don't want any exhaust leaks.
Step 7. Plug it back into the black female O2 sensor cable found on the drivers side.
Step 8. If your wrx already threw a CEL at you, then you need to reset your ecu. To do this all you need to do is grab a 10mm socket or wrench and disconnect the negative end of your battery. Depress the brake for roughly 5-10 seconds (i have never needed to depress the brake any longer than this in order to drain the leftover charge flowing through my car) and then reconnect the battery.
Enjoy your clean instrument panel!
I'm not a professional auto mechanic, so If I have misinterpreted anything or explained anything incorrectly, then please PM me with any contradicting references that you have come across and I'll fix it pronto.
I have always used forums for "how-to" articles and I have never made one, so I felt it was time to start giving back! I hope this is a good start.