[SSnowflake84]

Hey everyone, so my 17 SS A8 a few weeks ago threw a P0420 code. Took to the dealer since the car only has 6,000 miles on it and they said it was a bad converter! I’m thinking how does a converter go bad that quickly, they replaced the converter under warranty. Had the car for 4 days for it to only throw the same code again! Took it back to the dealer and they had it 2 days and called and said the lead technician was having problems diagnosing the problem. They said they were going to get GM hq involved, as of today they said that GM recommended a new ECU calibration. So after uploading the calibration the tech drove it on and off all day and no code has came back. Car is suppose to be ready for pickup tomorrow as long as it’s good tomorrow morning. They still don’t know what caused the car to mess up. I do have the GM CAI with the GM flash and asked if it could have been the flash and they said they didn’t think so.

My question to you all on here is what do you think may be the problem? The only other mod is a set of Hooker Shorty headers that was installed at the dealer when the car was brand new, it was part of some performance package they do on the cars. I don’t think that would have an affect on it since the stock manifolds are Shorty’s anyway. Car seems to run just fine each time the engine light was thrown, I have no oil consumption of any kind or noises from the motor. I hope a recalibration doesn’t make the car feel slower or drive any different than before. Just looking for some advice! Thanks for the help.

[TJay74]

While shorty headers typically are not supposed to change the location or the primary cats, they could change the location a minimal amount and along with being more efficient cause the cats to not light off soon enough.

From the time you start up and drive off there is a background timer running in the ECM, it is timing how long it takes the car to go from open loop mode to closed loop mode, then a certain time after that it expects the cats to start lighting off and the rear O2 sensors to respond the cats are working at the correct efficiency.

I think the time from OL to CL is 90 seconds or so, if after soo many drive cycles the ECM sees the primary cats are not responding correctly and lighting off it will trip the P0420/P0430 catalytic efficiency below threshold code.

[SSnowflake84]

Originally Posted by TJay74

While shorty headers typically are not supposed to change the location or the primary cats, they could change the location a minimal amount and along with being more efficient cause the cats to not light off soon enough.

From the time you start up and drive off there is a background timer running in the ECM, it is timing how long it takes the car to go from open loop mode to closed loop mode, then a certain time after that it expects the cats to start lighting off and the rear O2 sensors to respond the cats are working at the correct efficiency.

I think the time from OL to CL is 90 seconds or so, if after soo many drive cycles the ECM sees the primary cats are not responding correctly and lighting off it will trip the P0420/P0430 catalytic efficiency below threshold code.

That’s a good explanation! I wondered the same thing myself, I even questioned that when I bought the car, if the headers would void the warranty or interfere at all with the stock tune. Like I said when I bought the car new it already had these mods on the car, which surprised me seeing as most dealers don’t install aftermarket parts and keep the factory warranty.