BigE

iTrader: (2)

A friend of mine is complaining about drivability issues with his 99 2 dr Tahoe (LT1/SBC PCM). He said it's getting 12 mpg (he says it's normally 18) and it has backfired slightly on him once. I couldn't discern any difference than I had noticed before. He did have a misfire code. I cleared it. I scanned the truck (HP Tuners) and the primary O2's behave as an O2 should, dither control - steady harmonic bouncing between 0 and 1000 mV. The after cat O2's did not follow this. They go high and stay. Then low and stay. Start to oscillate and then go flat again. Is this characteristic of the secondary O2's? Any ideas?

Thanks