p0420 help

[FinalBreath05]

New member here. 03 Yukon XL Denali. I've got a P0420 and I also get a flashing cel anything above 2400 RPM's. I've replaced plugs, wires, coil packs, cam sensor. Cleaned MAF sensor. Replaced all my exhaust manifold bolts. I had 3 broken on each side. Any help would be appreciated.

 

[Scottydoggs]

thats the old bad cats code. they need to be replaced.

 

[PG01]

http://www.bigdaddiesgarage.com/mini-cat-cel-fix.html

https://www.obd-codes.com/p0420

 

[Rocket Man]

I agree, the cats are probably plugged. That's why the cel is flashing too. Don't continue to drive it with the light flashing or you will do serious damage.

 

[swathdiver]

Did you do all that stuff because of the P0420?

 

[FinalBreath05]

The only thing I replaced because of the code was the cam sensor. The rest was just a tune up. Truck has 330k on it. I've been driving it the better part of a year with the CEL on and I just keep it out of the RPM range where it'll flash. I've got a scanner but haven't hooked it up yet. I replaced the exhaust manifold bolts because it was leaking and sounded like crap.

 

[swathdiver]

Ok, well Matthew, a P0420 has nothing to do with a camshaft sensor. That's the driver's side cat all day long. I've been having the other one act up on me for about six months now and just the other day the driver's side threw a code. Lord willing, they'll be in the trash later this week.

After you pull the codes share them with us and we'll do our best to help you out. It's cool to see trucks with that kind of mileage still on the road. Came across an old-timer a month or so ago with an old Yukon with the body and interior near immaculate with that many miles and it sounded just fine too.

 

[FinalBreath05]

Ok, well Matthew, a P0420 has nothing to do with a camshaft sensor. That's the driver's side cat all day long. I've been having the other one act up on me for about six months now and just the other day the driver's side threw a code. Lord willing, they'll be in the trash later this week.

After you pull the codes share them with us and we'll do our best to help you out. It's cool to see trucks with that kind of mileage still on the road. Came across an old-timer a month or so ago with an old Yukon with the body and interior near immaculate with that many miles and it sounded just fine too.

I replaced the cam sensor because of the Flashing CEL. This was a retired gentleman and him and his wife traveled a lot. Trucks in immaculate shape. I couldn't pass it up. Only two codes I have is P0420 and a P0300

 

[swathdiver]

The latter code is generic, so take care of the cats first. Here's your chance to get long tube headers and a catless y-pipe for $200 if you don't have to worry about emissions testing.

 

[Rocket Man]

The P0300, multiple random misfires, is probably because of the cats too. If the engine can't breathe it can misfire like that. I'm sure you notice it if you try to get it above that 2400 RPM, and if you continue to push on the gas it will buck and misfire badly. That's ******* your engine even at lower RPM's.

 

[FinalBreath05]

The P0300, multiple random misfires, is probably because of the cats too. If the engine can't breathe it can misfire like that. I'm sure you notice it if you try to get it above that 2400 RPM, and if you continue to push on the gas it will buck and misfire badly. That's ******* your engine even at lower RPM's.

It seems like it runs perfect. I can't feel a misfire at all. But I'm going to order a cat and install it next week.

 

[Rocket Man]

It seems like it runs perfect. I can't feel a misfire at all. But I'm going to order a cat and install it next week.

What happens when you try to get it to say 3500-4000 rpms? Besides the flashing cel light. When I had this same problem (one cat was eventually diagnosed as 65% restricted and the other at 50%) I started throwing all kinds of codes related to misfires, tps, maf etc if I stepped on the pedal hard and it would buck and cough and sputter. It was hell trying to get on the freeway. BTW if one of your cats is bad the other isn't far behind and both really should be replaced at the same time to keep both sides breathing the same.

 

[FinalBreath05]

What happens when you try to get it to say 3500-4000 rpms? Besides the flashing cel light. When I had this same problem (one cat was eventually diagnosed as 65% restricted and the other at 50%) I started throwing all kinds of codes related to misfires, tps, maf etc if I stepped on the pedal hard and it would buck and cough and sputter. It was hell trying to get on the freeway. BTW if one of your cats is bad the other isn't far behind and both really should be replaced at the same time to keep both sides breathing the same.

The flashing CEL goes away. It only flashes in the 2400-3200 RPM range.

 

[Rocket Man]

That's weird I have to admit.

 

[Steve A]

Check your O2 sensors, they can cause that 0420 code as well.

 

[swathdiver]

How so Steve?

 

[Scottydoggs]

How so Steve?

its the o2's that are reporting the cat code, they are what reads the exhaust gasses, so if one is lazy or gone bad, like you have a code for one, that could cause a cat code.

if your o2's are over 80,000 miles old, they could use replacing. they dont last forever. by 80k they are no longer as good as they could be.

 

[swathdiver]

But O2 sensors have their own codes for when they fail, this is what I do not understand.

 

[Scottydoggs]

But O2 sensors have their own codes for when they fail, this is what I do not understand.

just because theres no code dont mean they are 100% good. they wear out by 80k.

my regal had a new ac delco o2 installed, all seemed fine till 6 months later when i was getting kr for no reason. did a scan and found my o2 voltage was dropping to nothing wot, and that will mess up the fueling. changed that o2 out, no more kr. chased my tail for a week over that one. had no o2 codes either. but that o2 was bad for sure.

 

[swathdiver]

Ok, I've had degraded O2 sensors before, used to change them out all the time with the turbo Buicks. They just never set off codes other than a failed sensor code. Live and learn I reckon.