Stumbles and dies when idling

[iCrash]

1999 Lumina 3.1 130k

 

Code P0420 which has been cleared and a few times and just came back. It hasn't been an issue until now. I've already replaced the O2 sensor after the cat and cleared the code but it came back. I noticed too that after the first couple minutes after startup the RPMs would fluctuate up and down around 1k and then finally settle. Now the rpms at startup are fine but its not until about 10 mins later of it idling the engine starts to stumble and then finally fall on its face. It starts right back up like nothing happened and drives fine (feels underpowered when stomping on it though). On my Torque App for Android I was monitoring my O2 sensors voltages while this was occurring. The graph was showing its normal oscillating thing and then as soon as the engine starts to stumble the O2 sensor reading flatlines.

 

What else should I look for on the Torque App and does this sound like its the cat? Thanks

 

I should also mention I was having a problem with a vacuum leak (I'm pretty sure I threw a code for it) and replaced the PCV tube from the intake to the valve cover.

[RobertISaar]

P0420 - catalyst low efficiency.

 

i would anticipate replacing the cat, especially with the comment of feeling underpowered, i would expect the cat to be plugging up. misfires can also cause this code to get set.

 

the PCM doesn't run that DTC test until the engine has been running for 10 minutes, which lines up with the timetable you've stated. the test modifies the fuel pulsewidth to go noticably rich for up to 7 seconds and then noticably lean for up to 10 seconds.

[Garrett Powered]

I have had the rad fan pull down the voltage so low that the car stalled.