Very Stumped by CEL Codes PO1276 and 1275, please help

[Eric Pinsker-Smith]

Hi All,

 

I have a 1999 C4, and i have really hit a wall with CEL Code 1276 ("aging of bank 1 upstream O2 sensor ahead of Catalytic Converter"). Last year, I got a new short block from Porsche, and attached to that a new FabSpeed exhaust X-pipe with new cats. They guarantee it will not trigger a CEL (and they say they have never encountered this issue before). 

 

My symptoms are as follows: If I drive on the highway for 45+ mins, on the following drive cycle, I get Code 1276 (and sometimes 1275 too). If I clear the code, it will not trigger unless i do some longer highway stints. When this first occurred, i replaced the upstream O2 sensors with new Bosch ones. When it kept happening, I also did the downstream ones. Then, I replaced the mass air flow sensor, after reading it can cause this issue. We smoke tested the exhaust to ensure its not leaking. We also flashed the ECU with Porsche's latest stock software, just in case there was an old tune or something causing this.

 

When plugged into a durametic, it looks like the upstream-to-downstream O2 sensor response time that's triggering this CEL is ~1.2 seconds (the CEL trigger threshold is less than one second). When using a gas like propane to test O2 sensor response, the response seems instantaneous. 

 

Does anybody have any thoughts on what i might do next? Im tearing my hair out on this one.

 

Thanks,

Eric

[JFP in PA]

From the looks of what you have posted, the aftermarket cats may not be doing the job.

[Eric Pinsker-Smith]

@JFP in PA i was thinking that too, but after diving in more, it seems that the aftermarket cats 1) have not caused this issue on the other 130+ vehicles they are on, and 2), the cats are LESS restrictive than the factory ones, which would serve to decrease the O2 sensor response time instead of increasing it. That's what has me so confused. 

[JFP in PA]

On way to find out is to take the car to a shop with an exhaust system "sniffer" and see if the car goes out of spec when the CEL is lit; if it is, the problem is the cats, regardless of however many other cars are fine.

[Eric Pinsker-Smith]

@JFP in PA not sure im following; the O2 sensor delayed reading implies that the cats are either leaking or not flowing air fast enough (clogged), but examining them closely with a scope doesn't reveal any clogs or anything. They are also new, and the fact that the code can trip on both banks makes me think this is not the case. Assuming the "sniffer" just sees that the exuast gasses meet emissions specs, which i think is what they would see, what would my next steps be?

 

Thanks,
Eric 

[JFP in PA]

Either the sensor are not telling the correct data, or the cats are not working correctly, the sniffer takes both out of the equation and tells you exactly what is actually coming out of the engine.  As you have new sensors, which appear to be working correctly, and without any change in the problem, running the sniffer will eliminate them and give you another perspective.  As Sherlock once said, once you have eliminated everything else, whatever is left is the problem.

 

And just because a part is new, doesn't preclude it being bad; we have seen many parts crap out right out of the box.............

 

And to eliminate exhaust system leaks, you could smoke test the system.