P0420 code will not go away!!!!!!

The Cat. has been changed twice. I believe they also changed the rear o2 sensor. The light just comes right back after about a day of driving.

By the way it is a 2000 Blazer ZR2 4.3L

Thanks in advance for any help!!!!!

Welcome to AF. Well, since the CAT was replaced twice, and you mention the rear/post cat sensor was replaced. If it actually was replaced and its still giving the a p0420, the next thing I would do is get a scanner, and watch and see what the 02 sensors are doing, both the pre cat sensor(s), and post cat sensor. If you can't really pinpoint anything out of the ordinary, you would have to get the truck on an gas emmission analyzer test and basiclly see what is coming out the tailpipe. This would have to be done both pre and post cat to see what readings you are getting, and you then should be able to figure something like this out. In my opinion, I would bet it has something to do with some engine sensor, probably 02, so getting a scanner would be my first mode of attack for this problem. Good luck, and let us know what you find.

Thanks. I had the Cat. done at midas and they have been helpful to this point but now he admits he is stumped. He actually said he would pay for the dealer to do a second opinion diagnostics. That is my next step as of now. I will keep this posted in case someone has a similar problem in the future.

Welcome to AF. Well, since the CAT was replaced twice, and you mention the rear/post cat sensor was replaced. If it actually was replaced and its still giving the a p0420, the next thing I would do is get a scanner, and watch and see what the 02 sensors are doing, both the pre cat sensor(s), and post cat sensor. If you can't really pinpoint anything out of the ordinary, you would have to get the truck on an gas emission analyzer test and basically see what is coming out the tailpipe. This would have to be done both pre and post cat to see what readings you are getting, and you then should be able to figure something like this out. In my opinion, I would bet it has something to do with some engine sensor, probably 02, so getting a scanner would be my first mode of attack for this problem. Good luck, and let us know what you find.

I agree, a scanner will help find the problem. There are many possible causes, and seeing what info the computer is seeing will help narrow it down. It could be a problem with the wiring harness. I've heard that some aftermarket cats have caused the code. It could also be the precat sensors. To determine the cat efficiency, the post cat sensor readings are compared to the precat sensor readings. If the precat sensors are providing false information, it could set the code erroneously.

The P0420 doesn't always mean the rear O2 sensor.

It means that a bank1 sensor is bad. So it could be the O2 sensor in the driver's side exhaust manifold.

Make sure they changed the actual sensor, you saying "you believe they did it" means you don't know for sure" means that it might be the old sensor and the problem all along.

The P0420 doesn't always mean the rear O2 sensor.

It means that a bank1 sensor is bad. So it could be the O2 sensor in the driver's side exhaust manifold.

Make sure they changed the actual sensor, you saying "you believe they did it" means you don't know for sure" means that it might be the old sensor and the problem all along.

I will remember to ask for the driver side exhaust sensor to be checked when it goes to the dealership for diagnostics. They did tell me that the first thing they would do is a reverse flow test? This I guess is to find out if the Cat that was installed is working properly. I am totally frustrated here. All I really want is to get my truck inspected!!!!!!!!!!!

Thank you for all of your suggetions and I will definately let you guys know the outcome.

This has nothing to do with the CAT at all.

These guys don't sound too professional.

This has nothing to do with the CAT at all.

These guys don't sound too professional.
P0420 is the code that the PCM sets when it thinks the cat isn't working efficiently. It is set when the PCM doesn't see a noticable difference between the precat and postcat O2 sensors. It's normally caused by a bad cat or a bad O2 sensor (precat or postcat) that's sending the wrong info. I've also heard of some high flow cats tripping the code.

This has nothing to do with the CAT at all.

These guys don't sound too professional.

I have talked to mechanics at 3 different places and they are all stuck on the Cat. I will have to say I am no mechanic I realize they need to be looking past this. The dealership sounds to be on the right track. I just wonder if they will give me the right info knowing I will not be getting it fixed there.

By the way I just talked to my cube neighbor and he says he has been through this same ordeal. He still has a light on after 2 Cat's...sounds familiar.
He actually took his to the dealership also and they told him it was the Cat.

I did hear a rumor that these after market Cat's do not function well sometimes?

I am so frustrated!!!!!!!!!!!

Highly doubtful that it is a cat problem.

I have had a highflow cat installed and I had zero problems.

The Cat. has been changed twice. I believe they also changed the rear o2 sensor. The light just comes right back after about a day of driving.

By the way it is a 2000 Blazer ZR2 4.3L

Thanks in advance for any help!!!!!

What is the mileage?
Cat codes can be caused by many things.
First thing to check is engine running/tune up.
Then grab a good engine capable scanner and go to work on checking all data and 02 sensor operation.

Good luck and a little info below.
http://www.troublecodes.net/articles/catfailure/

DTC P0420, P0421, P0430 & P0431: Check Possible Cause Of Misfire DTC P0420 and P0421 indicates bank one catalyst system efficiency is minimum requirement. DTC P0430 and P0430 indicates bank 2 catalyst system efficiency is minimum requirement. Possible causes are as follows: Use of leaded fuel. Oil contamination. Cylinder misfire. Fuel pressure too high. HO2S sensor improperly connected. Damaged exhaust system component. Faulty ECT sensor. Faulty HO2S. Ensure ignition timing is correct. Retrieve all Continuous Memory DTCs. If misfire code(s) is not present, go to next step. If misfire code(s) is present, isolate cylinder and repair as necessary. Check HO2S Monitor DTCs If DTCs P0136, P0138, P0140, P0141, P0156, P0158, P0160, or P0161 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. Check ECT Sensor DTCs If DTCs P0117, P0118, P0125 or P1117 were present in step 1), service as necessary before continuing. If none of these codes are present in step 1), go to next step. If any codes except P0420, P0421, P0430 and/or P0430 were present in step 1), service as necessary before continuing. If no codes except P0420 and/or P0430 were present in step 1), go to next step. Check Rear HO2S Wiring Harness Turn ignition off. Ensure HO2S wiring harness is correctly routed and connectors are tight. Repair or replace as necessary. If wiring harness and connectors are okay, go to next step. Check Fuel Pressure Turn ignition off. Release fuel pressure. Install fuel pressure gauge. Start engine and allow to idle. Note fuel pressure gauge reading. Increase engine speed to 2500 RPM and maintain for one minute. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, go to CIRCUIT TEST HC . Check For Exhaust System Leaks If exhaust system leaks, it may cause catalyst monitor efficiency test to fail. Inspect exhaust system for cracks, loose connections or punctures. Repair or replace as necessary. If exhaust system is okay, go to next step. Check For Exhaust System Restrictions Inspect exhaust system for collapsed areas, dents or excessive bending. Repair or replace as necessary. If exhaust system is okay, go to next step. Ć Check Manifold Vacuum Install tachometer. Connect vacuum gauge to intake manifold vacuum source. Start engine and raise engine speed to 2000 RPM. Manifold vacuum should rise to more than 16 in. Hg. If manifold vacuum is okay, go to next step. If manifold vacuum is low, go to step 11). Leave tachometer and vacuum gauge connected. Start engine and raise engine speed to 2000 RPM. On a non- restricted system, manifold vacuum should quickly rise to normal range as increased RPM is maintained. On a restricted system, manifold vacuum will slowly rise to normal range as increased RPM is maintained. If manifold vacuum is okay, no indication of exhaust leak or restriction has been detected and testing is complete. If manifold vacuum is low or slow to respond, go to next step. Leave tachometer and vacuum gauge connected. Remove exhaust pipe from exhaust manifold. Start engineand raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is downstream from exhaust manifold. Reconnect exhaust pipe to exhaust manifold and go to next step. If manifold vacuum is still low or slow to respond, fault is in exhaust manifold or intake manifold gasket. Repair or replace as necessary and repeat QUICK TEST. Leave tachometer and vacuum gauge connected. Disconnect muffler/tailpipe assembly from rear of catalytic converter. Start engine and raise engine speed to 2000 RPM. If manifold vacuum is now okay, fault is in muffler/tailpipe assembly. Repair or replace as necessary and test drive vehicle to verify elimination of symptom. If manifold vacuum is still not okay, fault is in catalytic converter. Repair or replace as necessary. Check tailpipe/muffler assembly for debris from catalytic converter. Test drive vehicle to verify elimination of symptom.

Wow That Is Alot Of Info!!!! Would It Be Insulting To The Mechanic To Print It Out And Hand It To Them? I Like To Try And Fix Things My Self But Diagnostics Are Way Over My Head. I Will Read This And See If There Is Anything I Can Check Myself.

Wow That Is Alot Of Info!!!! Would It Be Insulting To The Mechanic To Print It Out And Hand It To Them? I Like To Try And Fix Things My Self But Diagnostics Are Way Over My Head. I Will Read This And See If There Is Anything I Can Check Myself.


Well it might depend on the mechanic.
But if they have not been able to fix it for you it sounds like they could us a little help.
The 420/430 code is one of the miss understood codes there is.
A lot of times No easy fix just a lot of hard work

Print it out and give it to him.
It will not do many more than hurt his feelings and It may help him.

If you are going to a dealer here is what to ask for.
GOOD LUCK AND LET US KNOW HOW IT GOES.

Tip on going to the dealer
Ask that only a factory certified gasoline engine performance specialist work on your car.
Not all dealerships are concerned with how repairs are dispatched, and not all dealership techs bother to take advantage of the training offered by factory (the vast majority of which is paid training). If the dealer is uncooperative, ask for your money back and call around for a dealer that will accommodate you.

I Passed!!!!!!!!!!!!

I Am Not Sure Really How But I Did. Like I Posted It Came Out Of The Shop With Another Cat. (#2). The Light Came Back On The Next Day. I Decided To Try Something That I Should Have Done In The First Place.

1. I Disconnected The Battery To Reset The Code.
2. I Changed The Airfilter (mainly Because It Hasn't Been Done In A While )
3. I Poured A Can Of Seafoam In My Gas Tank And Drove It For About 50 Miles.

As I Posted On Another Thread The First Time I Went To Get The Inspection It Would Not Communicate. Today Everything Was Fine And I Passed. It Could Have Been The Second Cat. But I Think It Was The Seafoam. Since The Light Came Right Back On After The Second Cat. Was Installed. I Realize This Is Just A Temporary Solution But Now I Can Relax And Try To Figure It Out If The Light Comes Back.

Thanks For All Of Your Input And I Will Post Again With Any Developements.

I WOULD DEFINATELY RECOMEND TRYING SEAFOAM FIRST IF YOU HAVE EMISSIONS PROBLEMS. IT COULD SAVE YOU ALOT OF TIME AND MONEY.

I Passed!!!!!!!!!!!!

I Am Not Sure Really How But I Did. Like I Posted It Came Out Of The Shop With Another Cat. (#2). The Light Came Back On The Next Day. I Decided To Try Something That I Should Have Done In The First Place.

1. I Disconnected The Battery To Reset The Code.
2. I Changed The Airfilter (mainly Because It Hasn't Been Done In A While )
3. I Poured A Can Of Seafoam In My Gas Tank And Drove It For About 50 Miles.

As I Posted On Another Thread The First Time I Went To Get The Inspection It Would Not Communicate. Today Everything Was Fine And I Passed. It Could Have Been The Second Cat. But I Think It Was The Seafoam. Since The Light Came Right Back On After The Second Cat. Was Installed. I Realize This Is Just A Temporary Solution But Now I Can Relax And Try To Figure It Out If The Light Comes Back.

Thanks For All Of Your Input And I Will Post Again With Any Developements.

I WOULD DEFINATELY RECOMEND TRYING SEAFOAM FIRST IF YOU HAVE EMISSIONS PROBLEMS. IT COULD SAVE YOU ALOT OF TIME AND MONEY.

You are not supposed to drop a whole can of Seafoam into your gas tank.

I bet money you still have a lazy O2 sensor causing the problem.

The Bottle Just Said That It Is Good For 10-25 Gallons Of Gas I Think. I Didn't See Any Other Directions. I Do Realize This Is Just A Temporary Solution But At Least I Got My Inspection And I Have Time To Try And Figure It Out.

what does this "seafoam" do for ur engine, i have heard a little bout it, but not a whole lot

Without Having The Bottle Here With Me The Best Answer I Can Give Is It Is A Cleaner. I Know That The Bottle Lists Alot Of Things It Claims To Do. I Also Know You Can Add It To Your Oil Also. Stop Buy An Autoparts Store And Check It Out. Make Sure You Read The Directions Though Apparently I Didn't Use It Correctly.

You might have cleaned something out.

I am using Redline SI-1 Complete Fuel System Cleaner for my truck right now.

Full bottle for a full tank and then after that tank is done you dose it with a small amount at each fillup.

It works, and does clean carbon out of the combustion chamber and the valves. Stopped a lot of tappin in my engine.

how many methods do you use to clean the engine in your truck?

dont you also run a PCV catch can, and do/did that water in the vacuum line deal to clean the engine out also?

how many methods do you use to clean the engine in your truck?

dont you also run a PCV catch can, and do/did that water in the vacuum line deal to clean the engine out also?

Yes, and what is your point?

just curious how you could say that Redline SI-1 is cleaning out carbon deposits and reducing tapping sounds from your engine, when you are using so many other "cleaning" methods in conjunction with it.

just curious how you could say that Redline SI-1 is cleaning out carbon deposits and reducing tapping sounds from your engine, when you are using so many other "cleaning" methods in conjunction with it.

1.) I haven't done a water decarbonization in over a year and a half.

2.) PCV catch can doesn't clean deposiuts in the combustion chamber, it keeps oil out of the intake and from burning with the fuel.

3.) The Redline stuff is really good and is the only off the shelf cleaner certified by Toyota and BMW.

http://www.redlineoil.com/whitePaper/15.pdf