P0420 Code

I have a 2000 Tahoe with the 5.3 Liter V-8. Its got 215,000 miles on it, and just started setting the SES light off. I pulled the codes, got P0420, Catalyst Efficiency Below Threshold. Can this mean anything other than my Cat decided it was time to head South?

I have a 2000 Tahoe with the 5.3 Liter V-8. Its got 215,000 miles on it, and just started setting the SES light off. I pulled the codes, got P0420, Catalyst Efficiency Below Threshold. Can this mean anything other than my Cat decided it was time to head South?


Nope, its definitely a bad cat.

Careful here with that code there are other issues that can cause that code, go to batauto.com and do a search on the tech articles there, very enlightening article about the code and it's diagnosis.

Careful here with that code there are other issues that can cause that code, go to batauto.com and do a search on the tech articles there, very enlightening article about the code and it's diagnosis.

VERY doubtful. I had this problem with my tahoe a couple years ago and it was explained to me from a few people that if you get this code it can be nothing else than a bad cat. I had my cat replaced and the problem was solved. what else could cause it? A bad O2 sensor??? Nope, they got their own codes and it would have to be messed up just right that it would cause this code and not set off a O2 code. Now, there could be something else causing the cat to go bad(but with 215,000 miles it could just be that its breathed its last breath) but I would bet money it is a bad cat.

The VCM determines that the oxygen storage capacity of the catalyst has degraded below a calibrated threshold

This diagnostic test is designed in order to measure the efficiency of the three-way catalytic converter (TWC) system. Catalytic convertor efficiency is a measure of its ability to store oxygen after converting the levels of hydrocarbon (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) to less harmful gases. The control module is able to evaluate the catalyst efficiency once the vehicle has met the enable criteria and the vehicle is at idle instead of the steady cruise speeds used in the past.

Once the conditions for running this diagnostic trouble code (DTC) are met, the control module commands either a lean or rich air/fuel ratio depending on the current state of the exhaust oxygen level. The control module issues a rich command if the exhaust is currently lean, or a lean command if the exhaust is currently rich. After completion of the first command, a second and opposite command is issued.

For example, if the control module were to command a rich mixture, the upstream heated oxygen sensor (HO2S) voltage would increase immediately. The rich mixture is delayed in reaching the downstream HO2S due to the conversion process occurring within the converter. The higher the efficiency, the more the delay before the rich or lean mixture is detected by the downstream oxygen (O2) sensor. As a result of the lower conversion efficiency within a damaged or poisoned catalyst, the delay in the rich or lean mixture reaching the downstream O2 sensor is significantly shorter. This DTC monitors the amount of time required for both the upstream and downstream HO2S voltages to cross a calibrated voltage threshold in response to the rich or lean command.
These conditions may cause a catalytic converter to degrade. Inspect for the following conditions:

- An engine misfire

- High engine oil or high coolant consumption

- Retarded spark timing

- A weak or poor spark

- A lean fuel mixture

- A rich fuel mixture

- A damaged oxygen sensor or wiring harness

And for those of you that use ACETONE

• The catalyst may have been temporarily contaminated with a chemical from a fuel additive, fuel contamination, or any of the above conditions.

GMMerlin; you are the man. Too many people jumping to conclusions about the OBD II trouble codes. I've read so many posts about the problems I've been having (P0300/P0420), and 90% of the people out there are going through the random replacement of associated parts, hoping (by process of elimination) to find the root of the problem. I've seen people replace both catalytic converters and still have the same problem $$$ !!!!

It is well and good to change regular maintenance items like plugs and fuel/air filters, but to start replacing things like all four 02 sensors, and catalytic converters without verifying that they are actually the root cause is ridiculous.

Keep up the good work here. With your help many of these motivated amateurs (like myself) can become skillful shade tree technicians; and save a lot of money doing it!!

GmMerlin... you are the man! Thanks for all your efforts and your great, detailed information. I am going through this same issue on my 1998 model(catalyst efficiancy) and I will let you know what I find. Although I have been out of the dealership for a few years now, I do have a degree in Automotive Technology and its great that you provide so much detail in your replys. Thanks ckmn

GmMerlin... you are the man! Thanks for all your efforts and your great, detailed information. I am going through this same issue on my 1998 model(catalyst efficiancy) and I will let you know what I find. Although I have been out of the dealership for a few years now, I do have a degree in Automotive Technology and its great that you provide so much detail in your replys. Thanks ckmn


TTT any fixes on this code yet? gmmerlin in one poast you say bad cat and in another you say to check a list of other things. ?????? I have a 99 tahoe and did a intake job on it and then took it to the shop to be timed and it ran like crap getting it there. They timed it and reset the comp. and off we went. My wife drove it 30 mi and engine light came on so I borrowed a scan tool and ran the code it gave me 2 codes p0420 and p0000? any ideas?


Thanks Rob

I don't see code p1000, some posts here seem to doubt the complexity of the p0420, if they were in the repair industry and saw the amount of cats being replaced and the code coming back they would be quite enlightened, that being said, GMMerlin's post on the code is on target. Possible coolant damaged the cat but only a graphing scanner or oscilloscope to compare pre and post cat sensors, or checking the inlet vs outlet temp or a warmed up cat will confirm if it is bad, there should be 100-150 degrees difference from the inlet to the outlet. I'd clear the code and see if it come back first.

TTT any fixes on this code yet? gmmerlin in one poast you say bad cat and in another you say to check a list of other things. ?????? I have a 99 tahoe and did a intake job on it and then took it to the shop to be timed and it ran like crap getting it there. They timed it and reset the comp. and off we went. My wife drove it 30 mi and engine light came on so I borrowed a scan tool and ran the code it gave me 2 codes p0420 and p0000? any ideas?


Thanks Rob
P0420 and P0430 are for a bad cat..the problem is that catalytic converters don't go bad on their own, something destroyed them.
On a very rare occasion I have seen an incorrect O2 sensor reading cause a P0420.
The complexity of the diagnosis (and the overlooked part) is the possible primary causes of the failure and their inspection and testing procedures prior to replacing the converter.
If you do not identify the root cause and correct it prior to replacing the converter, the new converter will fail also.

Well.... Here's the poop... The Bank 1 and Bank 2 catalytic converters have been gutted as it turns out. After checking the O2 signals on the front and rear O2's, and cracking the exhaust system open at the center gasket, I found that they had been punched out. So I am faced with 2 options: replace the cats @ about $300, or install a O2 sensor signal simulator. I am going to give the simulator a try; all my readings and research says that this should work. Anybody had any luck with any of these? I will post back when I get it up and going. I found that plenty of racers use these on their street/track vehicles, and they seem to work well.. We'll see. C

GMMERLIN, this is guided to you and your notes.

I see at the end of your list where you typed "• The catalyst may have been temporarily contaminated with a chemical from a fuel additive, fuel contamination, or any of the above conditions."

I got the code P0420 only a few miles after adding some fuel additive. When you say "may have been temporarily contaminated" does that mean there is a possibility of the light going out and the problem going away?

GMMERLIN, this is guided to you and your notes.

I see at the end of your list where you typed "• The catalyst may have been temporarily contaminated with a chemical from a fuel additive, fuel contamination, or any of the above conditions."

I got the code P0420 only a few miles after adding some fuel additive. When you say "may have been temporarily contaminated" does that mean there is a possibility of the light going out and the problem going away?

hey jim the greek ! gm merlin is very much gone forever.

this post is also very much too old for any reply.

welcome to these forums .

After reading some posts on this thread I cancelled an appointment at the shop to get the cat replaced, clearde the code, drove a while and havn't seen it return. Saved me some $$$