P0420 and P0430 2002 Yukon - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**truckster** · 11-23-2022, 12:20 AM

I have a 2002 Yukon XL with a 5.3 and automatic transmission. I couldn't get it to pass emissions after a tuneup (plugs, boots, two coils, cleaned MAF, replaced air filter, oil & filter change, fuel filter, Techron injector cleaner), so I replaced the catalytic converters and the rear O2 sensors. The O2 sensors are still reading low voltage (around 0.2v) but they're reading pretty steady. I'm not sure where to go next.

**mattfranklin** · 11-23-2022, 01:35 PM

These are all very reasonable changes. They're what I would have done.

I have to ask a super dumb question, though (because everyone's level of experience with codes and code tools is different). You cleared the codes first, right? And then the codes keep popping up after being driven a while?

One additional source for trouble code troubleshooting steps (at least for a couple of Hondas I used to work on) is going to the AllDataDIY.Com website. It's about $20 per vehicle per year. Not sure if GM does the step-by-step flowchart thing in their version of the shop manual, but the Honda one was pretty detailed.

On one YouTube on a Ford (https://www.youtube.com/watch?v=-ZWKablBkwE&t=127s) suggested that cracked intake or coolant leak into the intake could cause those errors.

Another YouTube (https://www.youtube.com/watch?v=zQ8BXAwK-QY) suggested it could be exhaust leaks skewing things.

Good luck and please post back with your progress. This is interesting stuff.

**truckster** · 11-23-2022, 01:48 PM

I cleared the codes multiple times; I have a pretty decent scanner. Watching the live data, everything looked OK. Fuel trim was good, the rate of change on the upstream and downstream O2 sensors looked good, no misfires, and I couldn't figure out why the codes kept coming up. I finally decided to use some anti-foulers on the downstream O2 sensors. I was able to get it emissions tested today.

Bigdav160 · 11-26-2022, 08:13 AM

Too much oxygen (low voltage) indicative of an exhaust leak. Broken exhaust manifold bolts?

Glad you got it to pass

**truckdude239** · 11-27-2022, 02:50 PM

whta brand of cats did you install? i made the mistake of ordering a $200 set of cat on my tahoe and got 8k miles out if them, ive had a dealer account have me install cheap cats that never worked out of the box.

**truckster** · 11-27-2022, 08:22 PM

They were cheaper cats. To be honest, as long as I get it to pass emissions that's all I care about.

BigDave, the only exhaust leak was the one where the two halves of the replacement cats meet where the stock crossover is located. The cuts for the flange are too deep. A little weld took care of it.

11-23-2022, 12:20 AM	#1
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,794	P0420 and P0430 2002 Yukon I have a 2002 Yukon XL with a 5.3 and automatic transmission. I couldn't get it to pass emissions after a tuneup (plugs, boots, two coils, cleaned MAF, replaced air filter, oil & filter change, fuel filter, Techron injector cleaner), so I replaced the catalytic converters and the rear O2 sensors. The O2 sensors are still reading low voltage (around 0.2v) but they're reading pretty steady. I'm not sure where to go next. __________________ I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson

11-23-2022, 01:35 PM	#2
mattfranklin Senior Member Join Date: Dec 2011 Location: Moraga, CA Posts: 800	Re: P0420 and P0430 2002 Yukon These are all very reasonable changes. They're what I would have done. I have to ask a super dumb question, though (because everyone's level of experience with codes and code tools is different). You cleared the codes first, right? And then the codes keep popping up after being driven a while? One additional source for trouble code troubleshooting steps (at least for a couple of Hondas I used to work on) is going to the AllDataDIY.Com website. It's about $20 per vehicle per year. Not sure if GM does the step-by-step flowchart thing in their version of the shop manual, but the Honda one was pretty detailed. On one YouTube on a Ford (https://www.youtube.com/watch?v=-ZWKablBkwE&t=127s) suggested that cracked intake or coolant leak into the intake could cause those errors. Another YouTube (https://www.youtube.com/watch?v=zQ8BXAwK-QY) suggested it could be exhaust leaks skewing things. Good luck and please post back with your progress. This is interesting stuff. __________________ (Very) Slow-Going Build Thread: Stock 1970 Short Step with Stock 1970 LT-1 http://67-72chevytrucks.com/vboard/s...d.php?t=567340

11-23-2022, 01:48 PM	#3
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,794	Re: P0420 and P0430 2002 Yukon I cleared the codes multiple times; I have a pretty decent scanner. Watching the live data, everything looked OK. Fuel trim was good, the rate of change on the upstream and downstream O2 sensors looked good, no misfires, and I couldn't figure out why the codes kept coming up. I finally decided to use some anti-foulers on the downstream O2 sensors. I was able to get it emissions tested today. __________________ I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson

11-26-2022, 08:13 AM	#4
Bigdav160 Registered User Join Date: Apr 2011 Location: Klein Texas Posts: 3,852	Re: P0420 and P0430 2002 Yukon Too much oxygen (low voltage) indicative of an exhaust leak. Broken exhaust manifold bolts? Glad you got it to pass __________________ My Classics: '72 K20 Suburban + '65 Dodge Town Wagon '72 Corvette Roadster +'67 Corvette Roadster '73 Z-28 Camaro '63 Ford SWB Uni Pickup '50 Ford Coupe

11-27-2022, 02:50 PM	#5
truckdude239 Senior Member Join Date: Sep 2005 Location: Lewisville, Nc Posts: 10,217	Re: P0420 and P0430 2002 Yukon whta brand of cats did you install? i made the mistake of ordering a $200 set of cat on my tahoe and got 8k miles out if them, ive had a dealer account have me install cheap cats that never worked out of the box. __________________ David fuller Ase Certified Mechanic Click here to help support our board!! 1971 Chevy c-10 under going a 4.8l LSx swap Build Thread http://67-72chevytrucks.com/vboard/s...d.php?t=421305 2007 Honda Accord my daily 145kmiles 2002 Honda Accord 4 door With 330k(sisters car) 2005 toyota Avalon 228k( brothers car) 2002 Sububran 5.3 245k 2000 Tahoe 5.3l 378 General manager for Marco's Carwash & lube

11-27-2022, 08:22 PM	#6
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,794	Re: P0420 and P0430 2002 Yukon They were cheaper cats. To be honest, as long as I get it to pass emissions that's all I care about. BigDave, the only exhaust leak was the one where the two halves of the replacement cats meet where the stock crossover is located. The cuts for the flange are too deep. A little weld took care of it. __________________ I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson



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