2003 chevy impala trouble code 0420

my 2003 impala with 3.8L 3800 series II eng has been put out a trouble code 0420 catalyst efficiency below threshold bank 1 i have resent the code and it takes about a week before it resets the car runs fine should i replace the the o2 sensor and which one would i replace thank you for any help you can give

how many miles on car? it could be the cat, could also be the o2 sensor after the cat is going bad. a good scan tool is needed to read the actual values of the sensors.

the car has 93000 miles it has not lost any power and the fuel economy is good the code has been there for 8 weeks it started trowing the code after my wife had the fuel injectors clean

The sensor after the cat is there only to monitor the cat function, it is not used to adjust fuel/air mixture. They are easy to change yourself. Autozone will loan you the tool for free, even if you buy the new sensor (about $90) somewhere else.

The sensor after the cat is there only to monitor the cat function, it is not used to adjust fuel/air mixture. They are easy to change yourself. Autozone will loan you the tool for free, even if you buy the new sensor (about $90) somewhere else.


hence the code p4020 for cat efficiency below threshold. the sensor may or may not be bad, there could also be an exhaust leak causing a false reading at the after cat sensor. there are a number of things, up to and including the cat itself being bad. is why i was suggesting finding a decent scantool or code reader that will display engine data. some autozones have the sligtly advanced code reader that they use for the "store" one, never hurts to ask.

Could possibly be the catalytic converter. Check to see if the 2003 is covered under the extended warranty by GM. They increased it to 10 years or 120,000 miles. I just had to have my cat replaced in my 2002 3.8L w/ 105k miles....was clogged and made car run like junk. I was able to get it done for free because I went in armed with knowledge.

Could possibly be the catalytic converter. Check to see if the 2003 is covered under the extended warranty by GM. They increased it to 10 years or 120,000 miles. I just had to have my cat replaced in my 2002 3.8L w/ 105k miles....was clogged and made car run like junk. I was able to get it done for free because I went in armed with knowledge.

would look into that, beats paying ~$800 for new cats from the dealer, or even ~$300 for new ones from a regualr shop.

For more info on the causes of this c,do a sight search very common code, plenty of info.

I changed the o2 sensor light came back on after three days I did call about the warranty its not covered i been looking at cats just dont know which one is the best

I changed the o2 sensor light came back on after three days I did call about the warranty its not covered i been looking at cats just dont know which one is the best

So the 2003 isn't covered? They must have changed the part that year or something. I looked up the GM Campaign for the cat and they only list 2001 and 2002 (both 3.4L and 3.8L). That is a bummer...you can try RockAuto.com for great prices on parts. I think they have a Walker direct fit catalytic in the $200 range.

its your cat plugging up ,if you have an enine compression tester take off your egr valve leaving the wires hooked up and set it aside stick the com. tester in one hole and plug the other some how then have a friend start the car and watch the pressure it should only be a few pounds or so ,mine was at 15 so i ordered a new cat before it messes up something else.