993 TT Cat removal "check engine light"
Thread Starter
Track Day
Joined: Dec 2001
Posts: 15
Likes: 0
From: Redlands CA
993 TT Cat removal "check engine light"
Any idea of what resistor to use to trick the computer to not trip the check engine light. It is coding out on the rear O2 sensor not getting the correct reading. Is their anything premade that will work?
Thanks
Thanks
RL Technical Advisor
Joined: May 2001
Posts: 11,870
Likes: 75
From: Portland Oregon
Hi Dave:
I've had luck with removing the secondary sensors and fastening them up under the car so they sample ambient air.
This is what we usually do when cat bypass pipes are installed.
I've had luck with removing the secondary sensors and fastening them up under the car so they sample ambient air.
This is what we usually do when cat bypass pipes are installed.
Three Wheelin'
Joined: Jun 2001
Posts: 1,276
Likes: 0
From: SF Bay Area
Dave - no doubt that Steve W knows far more than I ever will on this stuff, but when I switched cat pipes I got the same reading. My mechanic merely cleared the check engine code with the Porsche diagnostic tool (the hammer?). The car seems to be fine with no check engine light since that time.
Steve - is there anything wrong with this approach?
Steve - is there anything wrong with this approach?
Instructor
Joined: Sep 2001
Posts: 204
Likes: 0
From: LI, NY
Steve,
I heard that one cannot use this trick with 996TT since both sensors readouts are used for computing the air-fuel mixture. Any comments on that? This may be quite relevant for many people with 100-cell cat converters.
I heard that one cannot use this trick with 996TT since both sensors readouts are used for computing the air-fuel mixture. Any comments on that? This may be quite relevant for many people with 100-cell cat converters.
RL Technical Advisor
Joined: May 2001
Posts: 11,870
Likes: 75
From: Portland Oregon
Rick:
Brain fade on my part; we certainly should have tried it as those sport cats were NOT catalyzing sufficiently.
Bill:
Sometimes that works, and sometimes the codes return in 50 miles or so. Each car seems to be a little different. If your mechanic was able to reset them and they didn't return, count your blessings,......
VS:
This is the same for all OBDII cars; all Porsches '96 and newer. All of these cars have 2 primary and 2 secondary oxygen sensors. This permits the ECU to read the differential lambda voltages that confirm proper cat operation.
When the cats go bad or cheap sport cats are installed, the secondary lambda values are too close to the primary values and the ECU will trigger the CE lamp and log a fault code.
Brain fade on my part; we certainly should have tried it as those sport cats were NOT catalyzing sufficiently.
Bill:
Sometimes that works, and sometimes the codes return in 50 miles or so. Each car seems to be a little different. If your mechanic was able to reset them and they didn't return, count your blessings,......
VS:
This is the same for all OBDII cars; all Porsches '96 and newer. All of these cars have 2 primary and 2 secondary oxygen sensors. This permits the ECU to read the differential lambda voltages that confirm proper cat operation.
When the cats go bad or cheap sport cats are installed, the secondary lambda values are too close to the primary values and the ECU will trigger the CE lamp and log a fault code.
Addict
Rennlist Member
Rennlist Member
Joined: May 2002
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The 993 TT ECU has some fairly complex software that detects the plausibility (i.e., form and frequency) of the waveform generated by the O2 sensor after the catalytics. See the factory OBD II service manual for more details. To me, it seems better to re-program the ECU software to check the plausibility of a signal with sport CATS or no CATS. Personally, I enjoy my car more when the factory engineering (e.g., OBD II diagnosis) can be preserved after a performance increase.
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RL Technical Advisor
Joined: May 2001
Posts: 11,870
Likes: 75
From: Portland Oregon
Hi Bill:
I agree with you,.....
I do not advocate removing the cats on any car and as we do software remapping on these cars, as well as others, I always retain full OBDII compatibility and and compliance. Its a really neat system, IMHO.
I agree with you,.....
I do not advocate removing the cats on any car and as we do software remapping on these cars, as well as others, I always retain full OBDII compatibility and and compliance. Its a really neat system, IMHO.
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Nordschleife Master
Joined: Jan 2009
Posts: 5,609
Likes: 84
From: MA
Old thread
Found this old thread on the oxygen sensors. Forgive me for bringing it up, but I wanted to know if the two oxygen sensors in 993tt work the way I think they usually work in cars.
This is what I think the typical late nineties early noughties car does. The first oxygen sensor is used to control the mixture on average at stoichiometric AFR ie lambda=1 when the car is in closed loop mode (and not in acceleration enrichment open loop mode). The ECU causes the actual lambda to hunt in quick cycles around lambda = 1, which one can see from the o2 signal. The catalytic converter stores and releases oxygen, working as a battery or accumulator of sorts. Thus, the second oxygen sensor after the catalytic converter should read steady Eddie lambda = 1 after the car has been in a closed loop mode for a little while and not see the hunting that the first sensor sees. If one removes the catalytic converter, the second sensor sees the lambda hunting in high frequency around one and the ECU figures the catalytic conveyer doesn’t work, has been gutted, or has been replaced by a test pipe.
This is how I think it works in a typical car, is it the same in the 993tt? Does the second lambda sensor test both lambda one and no hunting or just no hunting? In cars where the ECU only tests for hunting in the second sensor (and thus has a narrow band second sensor), leaving the second oxygen sensor in open air fools the ECU. If the system is smarter (and uses a wideband second sensor), that trick doesn’t work.
This is what I think the typical late nineties early noughties car does. The first oxygen sensor is used to control the mixture on average at stoichiometric AFR ie lambda=1 when the car is in closed loop mode (and not in acceleration enrichment open loop mode). The ECU causes the actual lambda to hunt in quick cycles around lambda = 1, which one can see from the o2 signal. The catalytic converter stores and releases oxygen, working as a battery or accumulator of sorts. Thus, the second oxygen sensor after the catalytic converter should read steady Eddie lambda = 1 after the car has been in a closed loop mode for a little while and not see the hunting that the first sensor sees. If one removes the catalytic converter, the second sensor sees the lambda hunting in high frequency around one and the ECU figures the catalytic conveyer doesn’t work, has been gutted, or has been replaced by a test pipe.
This is how I think it works in a typical car, is it the same in the 993tt? Does the second lambda sensor test both lambda one and no hunting or just no hunting? In cars where the ECU only tests for hunting in the second sensor (and thus has a narrow band second sensor), leaving the second oxygen sensor in open air fools the ECU. If the system is smarter (and uses a wideband second sensor), that trick doesn’t work.
Last edited by ptuomov; Aug 1, 2022 at 12:57 PM.
