What is the code P0420?

[Yooformula]

I know it came up as the catalyst code but what does this mean exactly? o2 sensor or the actual converter? Anyone have first hand experience with this? It was on our 03 Yukon with a 5.3l and 177k miles btw and I replaced both upstream o2's this past summer.

[Car Guy]

Odds are the converter has seen better days and needs replacing......

[spooln30]

^agreed. I read a while back about using battery acid to clean out cats. And I did try it on one of our shop cars to see if it works. And four months later no codes popped on. So I would say try that first as long as it's not broken or melted it should work for a quick and cheap fix. So take some old batteries and take the cat off and run the acid through it and done. Or just buy a new one.

[BAD LS1]

Catalyst System Low Efficiency, Bank 1 (driver side)

Aka your cat is fuckin' up. Hollow'em out and ill turn your codes off.

[Reverend Cooper]

more thank likely a result of the bad upstream o2's

[BAD LS1]

I can tell if the sensor by slow switch points, the post cat sensor that is which determines the cat efficiency. The pre-cat sensors were plenty sharp at least when i looked at it months ago.

[Yooformula]

yes they were and they were replaced shortly after but perhaps a clogged or going bad cat is causing my crappy gas mileage.

[wrencher]

The catalyst efficiency test are done with O2's.
They only test oxygen content in exhaust.
The cat's are going when they loose the ability to use & store O2.

That milage those converters are likely done. Both sides.
If it set a 420 bank 1, a P0430 is prolly not far off.

Definately an expensive fix to replace the cats. Tune them out is an option ???
Also have put 18mm spark plug antifouler's on the post cat O2's.
it shrouds the tip & helps them pass. Works OK most of the time on my beater car & few others.

[Coops Brother]

When the P0420 code sets it is only because the 4 O2 sensors are functioning properly and they have determined that the converter is not performing optimally, and needs to be replaced.....OHH, and not to pee in your oatmeal, both of the converters are in the
Y-pipe, you get to replace them both.

[Yooformula]

thanks alot guys!! I will end up simply replacing the ypipe with an off road pipe and have Tom remove the rear o2 via tuning.

[Reverend Cooper]

will it pass emissions like that or dont you care

[Yooformula]

I dont care about emissions! It just passed emissions plus when its time to renew the registration I would just move it to another location that doesnt require testing.

[Holeshot]

It will pass. My rear O2's are simmed out with HP Tuners and its never failed

[BAD LS1]

Yup it will pass the plug in test no problem.

[jakedrew]

if you dont have a way to remove the codes for good for the 2nd o2 sensors, Spark Plug anti foulers work good ;)

[GTO RLY?]

You'll pass if they don't put it on the rollers, which I haven't seen them do in a long time.

Of course, I haven't been to the sniffer man in several years... :D

[brotherbenn83]

They don't do that kind of emissions testing anymore. Anything pre-OBD2 gets a pass. Anything that HAS OBD2 gets a quick scan for codes, they don't even pressure test your gas cap anymore which was a waste of time/redundant test in the first place.

[Yooformula]

When I ran the truck last summer after buying it, they did pressure the gas cap and the last time the Formula was in an emissions station about 4 years ago, it got put on the rollers but still no visible check of anything and since codes were turned off nothing ever showed up on the scans.

[lilws6]

2 years ago when i had to take the T/A in for emissions they just scanned it didn't look under the car did the gas cap. and then left it run to tell me it passed and i left but the station reaked of fuel from my car not being tuned yet and having no cats :) i got simms on the rear plugs for now