TheOtherEric

Just wondering if anyone has a solution for running cat bypasses on an OBD2 car without getting a CEL.

I suppose if there is a solution, maybe it would also defeat the dreaded SAI CEL problem on OBD2 cars. (EDIT: no actually the issues are totally different...)

Robin 993DX

I was just talking to Chris Walrod this weekend about this while we are at lunch. I was under the impression that everyone just tie their 2nd set of o2 sensors on the side under the engine and it will work. But my question to Chris was that I can't imagine that it will work, because I thought the DME is looking for a voltage fluctuation with the 2nd set of 02 sensors.

TroyN

The two scenarios result in different codes so different solutions.

Only the pre-cat sensors are needed with cat bypasses. The post-cat sensors just are zip-tied and sit in clean air. I had Fabspeeds and no issues w/ CELs.

TheOtherEric

Ok, so you can run a cat bypass on OBDII cars by simply zip-tying the post-cat sensor in clean air? I've heard the idea, but haven't heard confirmation that it works until now.

NP993

It's what I've done for two years and no problems...

993inNC

It may. My car went 2 years without throwing a CEL then it started one day. The DME is looking for a variation between pre and post O2's and just tying the O2's up out of the way causes the DME to read "lean" since the O2's are reading clean air and giving an indication that the motor is running lean. That's why i suggest that cab owners not do bypass pipes, the smell will get overwhelming after the DME catches up with what's NOT going on and goes to full rich.
Its been rumored that the scoobie doo guys run a spacer for the post O2's that somehow allows for the right signals, but......

You could run sport cats that would give power gains and the legality side of fun. Be careful if you decide to go 100 cell, they may not pass emissions (we only do 200)

Alcaid

Picking up an old thread instead of making a new one.

So have anyone tried spacing out the 2nd set of O2 sensors while running cat bypass?

Do the 2nd set of O2 sensors control fuel mixture in any way or is the 1st set fully in charge of that alone?

TheOtherEric

Tons of non-porsche folks run spacers. Just google oxygen sensor spacer and you'll see what I mean. For that reason, I'd suspect it works in our cars as well but I haven't seen confirmation here.

Dunno if the 2nd set of O2 sensors affect mixture but it would surprise me since their readings will always be affected by the cat and it's current condition.