I have an 04'' Alero, I had the O2 sensor changed last week, and the service engine soon light has came back on. My code reader shows code P0420 "Catalyst System Efficiency Below Threshold". What does this mean and could it have something to do with changing the O2 sensor? [img]smileys/smiley5.gif[/img]

I think the PCM uses the O2 sensor to decide whether the cat is working properly, among other things. If I recall correctly, there is a forward sensor, and a sensor aft of the cat. Check there first. Also, I have heard that some brands of sensor do not work well with our cars. What brand did you get?

It was the rear sensor, and it was changed by the dealer. Not sure what brand of sensor.

They probably put an OEM part in, so that''s probably not the problem.


What caused you to get it changed in the first place?

I had the SES light on. The code was P0141.

Both codes relate to the rear o2 sensor. It is used only for measuring the output from the cat. I would look carefully at the wiring from the rear sensor, you could have a short or some corrosion in the connector or elsewhere.


It''s also possible that you got a bad o2 sensor, but less likely.

If the sensor is working does this mean I need to replace the catalyst?

Possibly. Have you noticed any other symptoms of a bad cat, such as rattling noises from that area, or loss of power or gradually worsening gas mileage?


If you suspect the cat, after a long drive (over 10 miles or so), as soon as you shut the car off, look under it at the cat. When my brothers cat went bad, it would glow cherry red after a long drive.


A mechanic can do a cat back pressure test for you, to diagnose a plugged cat. it shouldn''t be more than about $80 for the test. A cat will run you a few hundred for an aftermarket part.

I do hear a rattling when I accellerate. Owned the car for a week havent ran a tank of gas yet. Will check and see if the cat is getting hot.


Thanks for the info. [img]smileys/smiley1.gif[/img]

Checked CAT, it is not getting hot. Going to take it to a guy this next weekthat will check both sensors to see if they are working properly. If they are working properlymy next move would be toreplace the CAT.