DrLoch

Sounds like you need to use a data logger to see what is really going on.

donbrew

Been data logging for 18 months now.

Blue_SS

Interesting. Were both readings in closed-loop conditions? I would think even if the later one was not, that the system would revert back to the earlier one that was...

I know. Kind of grasping at straws here.

DrLoch

Have you seen the change in the post O2 when it threw the code?

donbrew

Closed loop, for the purposes of this particular discussion, the alcohol % is what puzzles me.

FWIW, what "they" say about the waves of the O2 sensors matching is what throws a P0420 is wrong. It is actually the equation:
Where:
1 = post cat response time - pre cat response time
2 = best failing OSC value from the calibration table (based on temp and exhaust flow)
3 = worst passing OSC value (based on temp and gas flow)

(1-2)/(3-2) = Normalized ratio

a Normalized ratio of 1 is perfect, a Normalized ratio of 0 is no cat at all.

The passing low Normalized ratio is either .35 or .50 depending on the alcohol content of the fuel.

OSC means Oxygen Storage Capacity.

This monitor is run after a certain drive cycle is run through, at idle, with the trans (auto) in drive. The A/F is forced lean then rich to obtain the response times.

Maybe I should cut n paste this into another thread sometime? The documentation for this can be found by following the link in my sig; https://www.chevyhhr.net/forums/how-...e-light-52828/

DrLoch

I agree, should start another thread with this information.