97 Malibu P0420 Cat code

I suspect a head gasket causing loss of anti freeze and P0420 code. If I fix the head leak is there any chance the cat will recover or must it replace it too?

116K miles 2.4 L

Antifreeze poisons oxygen sensors. Poisoned oxygen sensors won’t read properly. The code P0420 comes from the readings, compared, from the oxygen sensors. In short, after the coolant leak is repaired, change the oxygen sensors, and see what response the catalytic converter has.

Antifreeze also poisons cats (and catalytic converters). Definitely get the head gasket repaired and then deal with the cat code. If there were no other codes present, I would put money on a dead cat rather than an O2 sensor malfunction. Also, do not just change the sensors after the head gasket repair. With the right tools, one can tell whether there is an O2 sensor problem or a cat problem.

Yah, I’ve heard I can use the antifreeze to get rid of cats in the area. Would you use a volt meter in the DC range or AC to monitor the O2 sensors. I have a 2 channel scope I could use to watch both cats at the same time.

A Google search using the words “oxygen sensor voltage” revealed that an oxygen sensor is a dc voltage generator. If you have a 2 channel or even a 1 channel scope, that would tell you immediately if it was an ac or a dc voltage. Since the output voltage is very low, less than 1 volt, I would expect that a 1X probe would suffice.

Duh, I should have known that. Well I’ve got my work cut out for me in the head gasket area first anyhow.
Thanks for everyones help.