Originally Posted by Zulfiqar

This is not specifically MB related, its on a chevrolet impala 3.8 my uncle has, it triggered a P0420 code.

I tested the sensor via torque app and I see the rear sensor to be at about 0.15v constantly, if I rev the engine it does move, show a bit of small waves to 0.4 and then comes back down. This is quite wrong IMO. I tried to richen the engine by squirting some carb cleaner in it and the voltage rises for about 20 seconds then goes back down. The front one shows wave form but a bit odd - in short interval the waves are like bumps, long term shows consistent switching.

When I richen the engine the precat sensor spikes and sticks to rich condition. similarly when I lean it down (vacuum leak) it goes low.

Is this a cat failure or a sensor failure for the post cat sensor?