high flow cats on a gen 4.

[PirateViper]

I was in a parade today at low speed and never threw a CEL......as soon as I got away out on the highway back up to speed I was digging it an then it threw a CEL....go figure?

 

[Sonoman]

I was in a parade today at low speed and never threw a CEL......as soon as I got away out on the highway back up to speed I was digging it an then it threw a CEL....go figure?

That's normal for an O2 sensor CEL-- some of the OBD tests only run at a steady-state cruise at highway speeds. When I've had O2 sensors go bad on other cars, they've always thrown a CEL when just settling in to a cruise on the freeway (I.E. about one minute into the highway drive).