2007 Cobalt SS/SC......Mods are GM Stage II with Stage III pulley, Injen CAI, ZZP midlength headers, high flow cat with 2.5 exit, Megan catback.

Looking to see if anyone has run accross this issue. When at a stop light for about a minute, the AFR as read on my UEGO with lean out to 16 - 17 with an irratic idle. Until I turned off the MIL for P0420 it would set a check engine light for P0420 and P0171.

I have disonnected the battery and will scan my STFT and LTFT as the car most certainly needs tuned (pull in 3rd has a reading of 10 - 10.2 AFR).

Any feedback would be appreciated.

 

10 - 10.2 AFR is normal with GM Stage tune.

P0420 = cat efficiency. If you have a high flow cat or no cat you are always going to get this code.

When sitting at the light are your coolant temps rising as the AFR is leaning out? Does the AFR correct itself or does it drop to 16 - 17 and stay there?

 

Yes, I knew what the P0420 is and I turned it off with HPT just after I started the thread. The AFR will correct itself. The irratic idle and lean 16 - 17 lasts for about 5 seconds then it goes stoic and idles fine. No issue with coolant temps and my IAT 1 and 2 are fine. There doesn't seem to be a problem but I was curious if anyone else has run into this same issue.

Thanks for the reply!!!!