Alright, I'm stumped on this one...help me guys!

[ptcfast2]

I'll keep it short and simple

1998 Lumina w/ 3.1L V6 has 221,000 miles. It threw a P0420 (at idle according to the freeze frame data) about 10 minutes after I really pushed the car hard tonight. The only thing new in the past 48 hours is some Chevron fuel system cleaner was put in the tank (first time with this brand). The type of "potent" fuel system cleaner has never been used on this car (its had other cleaners like Lucas or Seafoam).

The catalytic converter is only 4 months old, and the oxygen sensors were replaced around the same time as well. After the code was thrown I checked the O2 sensors for basic performance. The pre-cat 02 sensor is functioning normally (adjusting as intended) and the post-cat sensor stays steady. No misfires, no hesitation, and car runs fine. The car passed emissions inspection at the end of May.

Anyone have any ideas (or dealt with this code before)?

[Airjer_]

What year and what motor?

Whats the post cat voltage?

[ptcfast2]

Originally Posted by Airjer_

What year and what motor?

Whats the post cat voltage?

Sorry I omitted the car year/engine info...posted this late last night: 1998 3.1L V6

Post cat voltage is around 0.1-0.2 at idle and stays there. Jumps up to 0.3 at times when cursing around but stays steady. The pre cat voltage is between normally 0.7-0.8 but I've seen it dip down to 0.3 when driving.

[Airjer_]

Here's what I would do. Run that tank of fuel down as far as you feel comfortable. Fill it back up, clear the code and see if it returns. If it doesn't than it was probably the fuel system cleaner. If it does than you can check into it a little further.

Cats on those years are fairly common to fail especial in tired engine (burning oil).

[Ruley73]

Check for any exhaust leaks near the O2 sensor. This can lower the O2 sensor's output voltage since it uses oxygen in the air around the outer shell to create the voltage. Obviously, exhaust gas contains far less oxygen than the atmosphere and less oxygen in the air = less output voltage produced by O2 sensor.