Hello there, longtime lurker, first time poster.

I have a SES light that came on a couple months ago. Pulled the code and yikes! P0420...

About my truck. 1998 GMC Sierra 1500 4wd Ext Cab SLE. 63,000 miles in pristine condition. It has the good ole 5.7 350.

Before this light came on for good, I had a periodic SES light and I can't remember the actual code, but it had something to do with the voltage of an O2 sensor. Now that code is gone and the P0420 stays on. If I clear the computer, it comes back on after 15-20 miles of driving. There has been no drop in power/performance from the motor, and the factory exhaust is in great shape.

I'm having a hard time believing that the cat could go bad at just 63k. Ive tried several things short of a new cat/o2 sensor. I did plugs/wires, dist cap/rotor, fuel filter, ect last week. The truck runs flawlessly.

The engine uses no coolant, and a very slight amount of oil if I'm pulling a trailer up in the mountains. (1/4-1/2 qt to an oil change)

Based on what I've written above, does it sound more like a bad downstream o2 sensor or the cat? I dont want to start throwing parts at it.

I'll also say that I'm in Idaho, a state that requires an emission test. I'm in the process of selling this truck to upgrade to a newer one.

Thanks very much in advance,
Andy in Boise.

 

I'm having a hard time believing your cat is plugged after 63k too.
I would guess your last O2 sensor is faulty.

Deffenitly a cheaper fix then a new catalytic convertor

 

If you have access to a code scanner that can read live data, do this for me. Get the truck at operating temperature, and run the truck and look at the O2 sensor data and watch the numbers. Watch all of them. The sensors should be switching from low to high, so like 100-700mV or so. This is not an exact figure, and the numbers don't need to be anything specific, but make sure the bank one and two sensors are both fluctuating between low and high numbers, indicating the sensors reading rich and lean conditions on each bank.

Then look at the sensor that's after the cat, probably bank one sensor two or something like that. The readings after the cat should be pretty steady, indicating a consistent burn inside the catalytic converter. If the readings on the rear most O2 sensor are similar to what the front O2 sensors are reading, then your cat isn't working.

If one of the front O2 sensors are reading a steady consistent voltage like the rear sensor, replace it. It is possible that the small amount of oil it burns has contaminated an O2 sensor or the cat.

Now I'll say from a lube technician's standpoint, EVERY engine burns a small amount of oil. And a 1/4 quart of oil is nothing to be concerned about if it looses only that much between oil changes.

Hopefully that helps solve your problem.

 

Guys,

I appreciate the replies.

This evening I decided to go to the parts store and replace the bank 1 after cat o2 sensor (Bosch brand). I then cleared the computer with the ODB2 scan tool. I took the truck on a 50 mile drive (mixed city and hwy) after I replaced the sensor. After the trip, the SES light has not come on yet. I'm going to take it on the same trip tomorrow morning to see if it stays clear.

I've got a buyer all lined up for my truck and I want to be 100% certain the probelm is fixed before I turn it over to the next owner.

If it comes back on I'll try monitoring the sensors via a scan tool.

Thanks,
Andy