On my '97 Z24 2.4 Auto, I get a random P0420 (Catalyst System Failure Sensor 1) message about once every few weeks. Had the O2 sensors checked and they are working well. Wiring to the O2 sensors is intact. I see no performance issues, so I reset the codes and am on my way, only for this to happen again randomly sometimes a few weeks later. Has anyone else had this happen before?

Just noticed wrong title - meant to be P0420, not P0240. Sorry about that!

i got the same thing... i think its gotta be one of the o2 sensors, or the cat converter... mine keeps tripping the same code over again and over again..clear it, week later it trips again..ive given up and just live with the check engine light..

this is the gas cap code aint it?



1989 Turbo Trans Am #82, 2007 Cobalt SS G85

Rodimus Prime wrote:this is the gas cap code aint it?

sorry its not...thats a P0440...........a P0420 would be, if im correct, catalytic converter efficency below threshold...........i had the the same issue i knocked on the converter and found that it was rattling on the inside......if its not and the O2 sensors are working properly i would bank on a converter..........who check the o2's and how well did the work....did anyone graph them and see how responsive they were....the first o2 should fluxuate above and below .5 v......the 2nd o2 should stay at a steady pace......not flucuating as much( more of a straight or gradually waving line)

lemme know if you have any questions