p0420 help

[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.