P0420 fixes?

randelli

Original poster
Member
Jul 9, 2012
4
Hi gang!

I tried this at another site with some success; but someone recommended that I try over here.

I have a 2003 Envoy SLT XL. My wife took the car in for a safety inspection and the following codes came up:

PO171
PO174
PO420
PO442

I have been researching the codes, and see a lot of suggestions - fuel pump, O2 sensors, cracked exhaust before the catalytic converter, vacuum leak, dirty gas, clogged injectors..... So it appears that I am running lean in bank 1 and 2, have a catalytic converter issue, and an evaporative system issue. You wouldn't know it to drive it because it seams to run fine- no sputterring or overheating. It does not smell like rotten eggs or leak oil, gas, or coolant.

We took the Envoy to a recommended mechanic for diagnostics and spent $195. the cost for repairs to pass smog: $2069.43 for a new catalytic converter ($1500 installed), clean mass airflow sensor, set monitors, replace air filter, replace fuel filter, new gas cap/gasket. These prices seem really wacked out to me. Am I out of touch, or are these OK costs?

So $125 later I have a new air filter, fuel filter, gas cap, oil change, education and dirty hands... I also ran a gallon of lacquer thinner mixed into 1/2 tank of gas to try to clean out the cat like I saw in this video Fixing Bad Catalytic Converters With Inefficiency Code P0420 - YouTube I checked the codes again and I only have that pesky P0420 catalytic efficiency code...

This is the catalytic converter assembly:

IMAG0917.jpg

IMAG0918.jpg


I can order a replacement for $490; but I would rather verify that it is not something else first. What else should I be looking at for this code before I dive in and buy a $500 cat? Could it be the O2 sensors? How would I test those to see if they are working, or clean them, before I buy those?

I am not so much a cheapskate; but in the past I have spent a lot of unnecessary money just replacing parts that did not solve the problem.

Thanks for your help!
 

Sparky

Member
Dec 4, 2011
12,846
$1500 to replace a cat sounds waaayyy too steep to me.

I'd sooner just get a universal cat of the right dimensions for $50-100 and have an exhaust shop cut the old one out and weld the replacement in for an hour's worth of labor than get a direct fit unit and have to screw around with exhaust bolts and other stuff that could be seized up and break off on you or whatever else those darned things can pull on you. Cheaper too.

If you can get the rear post-cat O2 sensors out, try swapping them side to side and see if the code changes to P0430 after several driving sessions. If it does, then the one sensor is the culprit (the one you moved from driver to passenger side). If it doesn't change and remains P0420, then the driver side cat is indeed not working. (Typically bank 1 = driver side, bank 2 = passenger side)
 

randelli

Original poster
Member
Jul 9, 2012
4
I will try this out, thanks for the tip!

I have read about checking voltages on the O2 sensors. What should my readings be? I have a DMM so that should be easy for me as well.
 

bluetbss07

Member
Jul 10, 2012
31
ya i agree my truck ALWAYS throws the p0420 code just because i dont have cats haha i just clear it out on my phone and move along but usually its one of ur cats or could just be a bad o2
 

randelli

Original poster
Member
Jul 9, 2012
4
Well my vehicle inspection sticker is due so I can't really ignore it or I would. Like I mentioned it runs well, doesn't smoke or smell, it is probably just a little bit off.
 

dingle

Member
May 26, 2012
59
from what ive read and understand the p420 will be thrown when the cat hits 94% efficient soits only lost 6% of its cleaning ability. i had my accord and my moms impala both throw the p420 in the last 6 months and both times it was the cat. i went and got a cat at advance auto the was a bolt in direct replacement for $153. and my moms was covered under warranty. and i had a muffler shop install it for 50 bucks. thats just my experience with that code.
 

randelli

Original poster
Member
Jul 9, 2012
4
Any tips on reaching and unplugging/plugging the O2 sensors in?

Yeah, stupid question, but for some reason all of my O2 clips are way high against the body in tight little spaces. Other posts I've read say "only took 15 minutes" - yeah right!!?
 

Irishboy02

Member
Apr 1, 2012
222
To me, i wouldnt rule out cats, however it also sounds like it could be o2 sensors. I would recommend changing those first, they need to be changed anyway from how it sounds. If the o2 isnt sending the right signal to the pcm, it is not getting a proper read of the exhaust gasses. The computer can assume it can be any of the issues you spoke of (vaccuum, dirty fuel etc). Taking care of the o2s first is also cheaper.

Id recommend try unscrewing the o2 before disconnecting it, then pull down on the wires to get some slack to help you out with the harness clip
 

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