P0420 bad cat?

About once a week or so i have to clear the check engine light for this cat inefficient code, ive heard a bad cat can cause this or a o2 heater can too but my question is when i turn the key to run the millivolts immediately start dropping down from the 450 volt bias but sensor 1 and sensor 2 are like. Mirror image of each other instead of sensor 2 being steady rich

Well, if I understand what you're saying, it does appear that the CAT is not lighting off. We need to determine why? Is it because it is bad or something it needs is not occurring?

Have you had a chance to verify that both forward and back O2s respond to a vacuum leak and propane?

Does the rear sensor ever look good even after driving for a while or after revving the engine for a while to warm up the cat?

Does your car have secondary air injection?

I would go straight for a bad cat on this, the downstream O2 showing an exact echo of a normal upstreamO2 hi-lo-hi-lo-hi-lo pattern when the engine is up to running temperature is the classic symptom.

No secondary air, i havent tried creating lean/rich conditions yet

The downstream is exactly following the upstream roller coaster --the upstream fueling is closed loop and giving the cat the ideal working conditions - as long as the coolant is fully up to temperature, raise the RPM to 3,000 or so for a minute or so than allow it to return to idle if the downstream returns to the same roller coaster pattern crossing the 0.5v mark every 1 or 2 seconds or faster the cat is dead ..... no point on testing further

How many miles, señor? Vehicle? Many cats are only good for 100-150,000 or 10 years of the vehicle life. Plus regional differences in fuel refining could mean your gasoline could be causing your issue. I have heard that 1 gallon acetone in a fuel fillup(~18-20 gallon) could remediate the issues of a failing catalyst. I have never tried it. When I got a repeated p0420 in my own car, I bought fuel from another source and the code went away. And that was 4 years ago.

The Thermoquad Cowboy