P0420 code on 2008 Prix

[guitarfish]

My '08 just hit 150K miles, and a little present appeared in the instrument panel, an orange colored engine light. (They should make it a green colored $ sign, because that's what it really means ).

Got it scanned tonight and it's a P0420. Did lots of searching on this board, it seems like it's the catalytic converter, or possibly the rear O2 sensor.

Not sure what I can do to troubleshoot. I'd love to hear experiences/suggestions.

Thanks!

NOTE - THIS ISSUE IS FIXED. See here for the result.

[Tech II]

What would help is if you have a scan tool that can plot the pre and post O2 sensors......when the cat is functioning normally, the pre O2 sensor is like a sine wave fluctuating from 0.2-0.8 volts, constantly changing.....

The rear O2 sensor should move slowly, staying high, then switching and staying low(depending on whether the cat is storing or using O2).....but it should switch nowhere near as fast as the pre sensor...


When the cat is bad, the waveform of the rear will mirror image the front one, and change just as rapidly.....

[olopezm]

Most of the time you can identify a bad cat converter because of a sulfur or rotten egg smell coming out from the tail pipe. Are you noticing any?

If the cat has clogged then you should be able to see it glowing red after a few moments of starting the engine; a clogged cat should also cause a lack of power...

Oscar.

[guitarfish]

I had a baf cat once before on a different Prix, with definite noticable symptoms. It was an '02 with one of the bad factory cats, which GM replaced. This time, the only symptom I have at the moment is the SES light. Thanks for the tips though, I need to do more investigating.

[GTP Dad]

If you have no driveability issues I would change the rear O2 sensor and see if the problem goes away. The rear sensor can go bad and will set a similar code about the cat being bad. If the problem persists I would have the cat checked.

[Tech II]

Kind of leaning towards GTP Dad on this one.....more often than not, if you have to replace the cat, you should replace the rear O2 also......so it couldn't hurt to do so......

[rkvons]

I was under the impression that when you start your car, two tests will be performed on the cat system. The first one happens right away and verifies that the pre and post cat sensors are reading about the same. The second test happens after the vehicle goes into closed-loop. This time the system checks to see if the readings are different between the two sensors. If they are not different enough, it fires the P0420 code. This then, points to the converter as being bad. Am I close?

[guitarfish]

I reset the SES light by pulling the battery. It stayed off for a few days (200 mi) then came back on. Yesterday I installed a new after CAT O2 sensor yesterday and reset the SES again, will see what happens.

I noticed the CAT is bolt on. Is this a job one can do at home? Also, should I stick with a GM CAT, or is there a quality aftermarket unit that's just as good?

Thanks

[GTP Dad]

A GM Cat is very expensive. I recommend an aftermarket cat. Try looking at Rock-auto.com for a replacement.

[guitarfish]

Oh well, 100 mi later the SES light is back. I guess it's time to have the CAT checked. PITA.

[olopezm]

Try erasing the codes with a capable scanner instead of disconnecting the battery (something I learned recently), maybe autozone will clear them for you.

I read somewhere they weren't doing any longer...

Oscar.

[guitarfish]

Originally Posted by olopezm

Try erasing the codes with a capable scanner instead of disconnecting the battery (something I learned recently), maybe autozone will clear them for you.

I read somewhere they weren't doing any longer...

Oscar.

Advance Auto wouldn't clear the code for me. Not sure what the big deal is. I reset it by pulling the battery cable for a couple hours.

[Tech II]

Disconnecting a battery does not clear a code.....There are code readers out there for 20-40 bucks when on sale....also you can buy a scan tool that will plot the pre and post O2 sensors for around 150(also reads data, codes, and clears codes).....Amazon.com has some good deals...

[guitarfish]

Originally Posted by Tech II

Disconnecting a battery does not clear a code.....There are code readers out there for 20-40 bucks when on sale....also you can buy a scan tool that will plot the pre and post O2 sensors for around 150(also reads data, codes, and clears codes).....Amazon.com has some good deals...

Well, on my car disconnecting the battery for a couple hours resets the SES light, which is what I needed.

[olopezm]

Originally Posted by guitarfish

Well, on my car disconnecting the battery for a couple hours resets the SES light, which is what I needed.

I used to believe the same, it works in my Ford cars but not on the GP and this is how I found it out. I had been getting a p0441 DTC since a long time, I replaced all rubber parts in the EVAP system and even the gas cap. Disconnected the battery a couple of times because I did other repairs and tested the car after each, and every time the CEL came on with the same code I was tired of looking for problems and since I don't have a smoke machine (nor I know a shop who has one) I decided to leave it alone until a couple of weeks ago when the same discussion about resetting the light came up, the guys wrote that disconnecting the battery would reset the CEL but wouldn't erase the codes stored in the PCM. That was when I hooked up my scanner, deleted the code and drove the car and voila! No more DTC's; unfortunately I still have a problem with the steering XD (and a bad cat converter).

Anyway, I hope my quick story helps understand the CEL thing .

Oscar.