RustyRotors

I recently installed a cat bypass pipe in my 96 993 C2. First I installed all four O2 sensors in the bypass pipe. Not surprising but after an hour long drive the CEL came on. Codes suggested a Cat problem. So I cleared the faults and removed the rear O2 sensor and "hung" it up inside the engine compartment so that it could breath some clean air.

No problem until I did a long drive down to California. After four hours of continuous driving I got a CEL again.

Where are you mounting the rear O2 sensors or what are you doing to overcome the CEL issue for Cat bypass pipes?

Thanks
Ron

TroyN

Hopefully you don't jinx me, but I have done what you did, hung them out of the way in the fresh air.

993inNC

This has been discussed before, but you need to ground them (keeping the O2's connected to the wire loop and sniffing) You got a bad cat reading because the system found no difference between the pre and post O2's. What you have created is a VERY lean mixture situation. Hopefully no damage has been done to the motor, in addition, it will take a little time to react to the new (grounded and sniffing) readings.

maximo993

.....is dangerous to the motor ? the next week i install a fabspeed catalytic converter bypass pipes ! i have only one oxygen sensor after and before.

TAI2

I kept the sensors connected and mounted on sturdy brakets 1/8" from the by-pass pipes and had no CEL problems for 1.5 years. I finaly did get a CEL and got the bad cat codes. Water/time/road dirt started wearing on the sensors and they eventually started to provide differing and "bad" O2 readings which eventually tripped the CEL. I ussually end up having to clean the sensors regularly and have avoided the CEL. But, no matter what I have done, I have always end up with a running lean issue. The smell tells me it is time to clean the sensors. I will end up putting the CAT back due to my pending NJ Emmisions test and the concern about engine damage due to engine damage.

Tom
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96 993 Cabrio - Black
21K miles
Fabspeed exhaust
Carbon fiber air box
Strut bar
Rennsport pedals
18" Turbo twists
One lead foot

993inNC

When the ECU sees rich on both sides of the "cat" (there or not) it leans the mixture out. The "bad cat" reading came from that situation, it saw the same reading and attempted to adjust probably until it was out of the prescribed range and then through the code when it couldn't compensate. Yes thats bad
The installation of a cat bypass isn't the problem, its the way it was installed that is.

RustyRotors

OK. just to be clear. My last install was for the pre-cat O2 sensors to be installed in the bypass pipe. This registers a pre cat sniff. The post-cat O2 sensors are still wired into the system and were left suspended in the engine compartment to sniff clean air. After four hours of continuous driving I got the CEL light.

Where did I go wrong? Still connected to the wiring harness is grounding them.

Thanks
Ron

993inNC

Ron,
You should clamp them to ground. The wire harness to O2 doesn't count if the O2 isn't grounded. I have sold quite a few bypass pipes in the last year and a half and to this point, no one has complained about a CEL after following that one direction. Try it and see what happens.

Felix

I thought the post-cat sensors were only there as part of OBD II to police the functionality of the cat to ensure it was cleaning up the exhaust and that the post-cat sensors have no effect on the mixture?

mr_bock

My Son used O2 sensor spacers (buy em at auto parts store) on the post sensors (had to drill them out) to move the sensors out of the exhaust gas path. This seems to work fine. Not sure if there is enough room in the P-car. This might be a solution...

Good Luck!!!!

RustyRotors

Thanks to everybody for their input. I will try to "ground" the post-cat O2 sensor. You are correct. They are connected and not addtionally grounded. So, after they are grounded I should 1) get no CEL and 2) the mixture (not too lean) will now be improved?

I did a 2400 mile drive with the CEL on.

Thanks again.
I will let you all know how it works out.

Did I mention it is LOUD without the cats.

Ron

RustyRotors

Just got the CEL code retrieved. It is P0136

Went on line and it says this code is "P0136 Oxygen Sensor Circuit Malfunction (Bank 1 Sensor 2)"

So for all 2500 miles I drove down to California and back I got one fault and that was after 4 hours of steady driving.

So if I have to ground them why did I not get a fault for the other side?

Thanks
Ron