PO 420 code

I have an S10: 4.3L vortec

I keep getting the 420 code (both ox sensors bad) even though I have replaced both oxygen sensors. Comes back about every 200 miles.

Any advice???

Thanks a lot,

Joe

A code 420 does not say 02 sensors are bad.

First thing to check for is tune up or anything to make it run rich or a engine miss.
Any blinking or check engine lights besides code 420?
What year and mileage?

http://www.troublecodes.net/articles/catfailure/


Here is a little info on 420 codes.
DTC P0420, P0421, and P0430 & P0431: Check Possible Cause Of Misfire DTC P0420 and P0421 indicate bank one catalyst system efficiency is minimum requirement. DTC P0430 and P0430 indicate 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 is not present, go to next step. If misfire code 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. 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 engine and 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.

The year is 98.

It's not actually my vehicle, it's my brothers. I'm just trying to help him track down the problem. I've gotten help here before so I asked him to get the codes read at autozone.

Either they told him the wrong thing or he got something wrong. I'll check and repost.

AZ can read codes but let there repair info go in one ear and out the other.
They are just parts people not trained techs.
Just get the free code check and post back code no.

If the code said P0420.
Go threw what I posted in upper post.
If any other codes or not a code P0420 post back code no.
With year and mileage.

Ok I called my brother. It's a 98. It has 220K miles. The engine was rebuilt @ 150K. He got a check engine light before and it was the O2 sensors. He replaced both with new plugs also.

He keeps getting the light though. This time it is the 420 code with no other codes set.

The info you posted is helpful but confusing because it keeps branching, but I believe the next step is to check harness routing and fuel pressure???

Hey will this work for the fuel pressure and vacuum checks?

FUEL PUMP AND VACUUM TESTER (http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93547) (pressure to 10 psi and vacuum to 28" HG)

Where do I take the fuel pressure reading?
Where and how do I take the tach reading?

What should the pressure and vac readings be for the tests above? (Go/No-Go ranges)


Thanks,

~Joe

It will work and is good for vacuum test.
But no for fuel pressure testing on yours.
You have a high pressure system on yours up into the 60 lb range.
You will have a schrader type valve test port on fuel rail for a test gauge to screw on to. Just like AC test ports on the old R-12 systems
The vacuum gauge is a good tool for testing engine vacuum and will tell you a lot about engine running and for testing for a restricted cat.
The Tach reading is the engine RPM's
Some good timing lights have a [TACH]rpm gauge built into them and engine scanners read RPM
But for the code 420 testing you will need a good engine scanner and other test equipment that gets into high dollars. low figure is 10K up.
Go to the link I posted on it and it will show some of the test done with a lab scope and scanner.
But as said make sure your engine is running up to par first.
And if more testing is needed I would suggest a good repair shop.

The link you posted was very helpful. I'm an electronic tech. and can do the testing myself with a little help. I'm obviously not a mechanic, but I do have an interest.

I can do the O'scope tests myself, but I see that other things could cause the same results as a bad cat. conv. Should I do the other testing first, to eliminate other problems? Does the high mileage indicate a good chance the cat. conv. is bad? (220K miles).

Also can you provide a pinout and/or color code for the O2 sensors?

Isn't the 0420 code "catalyst effeciency low"? Replace the cat, $152 at advancd auto.

I don't want to replace it until I am almost certain it is the problem.

The link you posted was very helpful. I'm an electronic tech. and can do the testing myself with a little help. I'm obviously not a mechanic, but I do have an interest.

I can do the O'scope tests myself, but I see that other things could cause the same results as a bad cat. conv. Should I do the other testing first, to eliminate other problems? Does the high mileage indicate a good chance the cat. conv. is bad? (220K miles).

Also can you provide a pinout and/or color code for the O2 sensors?

Always make sure engine running good first.
A miss fire ,poor tune up or rich running engine is one of the main causes of cat codes and failed cats
A dual channel lab scope is very good for testing.
Actually what sets the cat codes is the PCM does not see any difference between the front and rear sensor on the cat.
Watch for more activity on front 02 sensor than rear 02 sensor.
Make sure the sensors are good and reading right.
If the cat is dead front and rear 02 sensors are good will read be the same.
A engine running rich will do the same thing if cat is overloaded from rich fuel.
Always test with the engine good and hot and after a long drive.

220K is high mileage on engine and cat both.
But I have saw cats last up into the 350K range or more.
MT

A engine running rich will do the same thing if cat is overloaded from rich fuel.
If the average V. on the front sensor is around 450mV wouldn't this indicate that the engine mixture is OK, and I can procede with the test.

A miss fire ,poor tune up or rich running engine is one of the main causes of cat codes and failed cats
He did have bad O2 sensors originally, and he said the engine was sluggish when he accelerated fast (before changing sensors). Is it possible the bad O2 sensors caused his engine to run rich and that caused his cat. conv. to go bad.

I'll probably do the tests tomorrow and post the results.

Yes average voltage of 4.5 is good.
You should see a lot of wide switching lean and rich and both left and right banks even.
The rear 02 sensor or cat checking 02 sensor should be less active with voltage closer to 4.5 volts all of the time.
If the 02 sensors have just been replaced I would clear code and drive it a while.
Even 500 or more highway miles.
A cat that has been overloaded from rich fuel will need some driving to clean itself up.
MT

I ran the tests today with the following results:

Right (passenger side) Sensor: ~.4V Avg.
Left (Driver side) Sensor: ~.43V Avg.
Sensor after the Cat. Conv. ~.67V Avg.

It actually has 3 sensors. Both before the Cat. Conv. were changed. The one after the Cat. Conv. wasn't.
The V. on the 2 sensors before the conv. definately varied faster than the one after, but the Avg. V. was higher on the sensor after the Cat. Conv. I did the test with the engine fully warm while at idle.

Advice???

Thanks,

Joe

The test should be done on the road at 2-3K rpm range.
At idle you will not get a good test.
Make sure there is not another sensor before cat.
96 and some use 4 sensors.
A left and right front sensors and a front cat and rear cat sensors.
Also check rear sensor and make sure it is plugged in and hooked up.
I have run into dummie sensors tied in or hooked up in place of the real sensor.
Does the pcm have a chip or has it been reprogrammed?
If all else fails replace rear sensor or swap if any front ones will work and plug in back there.

The test should be done on the road at 2-3K rpm range.
At idle you will not get a good test. D'OH!!!
I didn't think about it at the time but the cat. conv. has to work a lot harder when the vehicle is actually going down the road.

There are only 3 sensors. The downstream sensor is an actual sensor and not a dummy sensor. Not sure about the PCM, he bought the truck used.