Diagnosis?

[ThreePines]

This code gets set ONLY when I lift off the throttle (or the cruise control does after I crest a hill) on long highway trips. Doesn't happen in everyday driving. The throttle is closed, and I assume there's only the idle amount of gas getting to the engine, though its in high gear and travelling 65+ mph. My truck is a 2001 Sierra 2500 HD w/ a 6.0L gasoline engine. 104K miles.

 

Resources I can find are general for this code, and refer to many likely causes. Anybody have some insights they can offer based on the limited situation where this occurs (closed throttle)?

 

Thanks!

[GM-Tech]

This code gets set ONLY when I lift off the throttle (or the cruise control does after I crest a hill) on long highway trips.

 

Sounds like you have a scanner? Can you pin down which cylinder is the culprit? What are the long term and short term fuel trims at when the problem occurs.