Hi,
2003 Monte Carlo SS 3.8.
Had a plastic burning smell about 1 months ago, off and on.
Thought it was comming from under the car, may have ran over
a plastic bag, caught on exhaust. Smell finely went away.
Then 1 week ago, the check engine light came on.
Took it to Orileys, they put the scanner on, came up with a code
check gas cap, coolant and oil level. The coolant was down 2 quarts
put that in, started the engine the check engine light went off.
Now 5 days later the check engine light comes back on.
Checked the coolant level again, all ok.
Any ideas whats going on?
Thanks for any help.

 

What's the actual code you are throwing??

 

I believe it was PO440.

 

Hi, The Maniac
The code is PO420.
Was not the one I thought it was.
Any Help.

 

P0420 is typically from a clogged catalytic converter.

 

The Monte Carlo, has only 53200 miles on it,
would the catalytic coverter plug up , or could it be
the O2 sensors, they have never been changed?
Any help.
Thanks.

 

Yes, it can be a clogged/failed cat and yes it can be the downstream O2 sensor. It can be as simple as a wire is damaged.

If it's not an OEM cat, any aftermarket cat runs the risk of popping that code (does not mean the cat is bad in those cases, the car is just not calibrated to it).

Don't get hung up on the mileage.

A poor man's test of a clogged cat is look under the car and if it is getting red, the cat is clogged. But you are not noting any performance issues while driving. So there stands a chance that the problem might just be an O2 sensor.

If it were me, if I was having no known performance problems, I'd replace the downstream O2 sensor and see what happens (as a DYI, it's the cheaper part). This one can be a bit of a pain due to tight spacing because GM wanted the thing to sit straight up and down (so you have to work in the "tunnel"). I've had to cut the wire and put a regular deep well socket on the O2 to get it out and had to select an arrangement to work out a breaker bar for tight spacing.

 

Thanks guys for all the help.
Maniac,
I will try your suggestion , and replace that downstream O2 sensor first.
If that does not do the job, I'll replace the cat.
I cannot afford a OEM one, I've looked at pros and cons on
Eastern brand Magnaflow, and Walker.
I think I will go with the Walker Brand.
Looks like better reviews than the other two.
Thanks again for all the help.

 

Understood on the cost of an OEM cat. Keep in mind, the non-OEM has been known to be a perfectly working cat and still throw the P0420 code. If that happens, I do offer a limited service in adjusting the PCM to delete that code. The link to the thread about that service is below:
https://montecarloforum.com/forum/pr...impalas-42773/

 

Exhaust shops can actually back pressure test your system and see if the cat is clogging. If you don't want to throw money at the sensor.