P0420 on a 2001 Pontiac Bonneville, diagnostic and correction:

Just spent a month and a ton of fuel trying to diagnose and correct a P0420 error so I could pass the emissions test, the SES light was lit.

First I cleared the error, and did a visual inspection of the oxygen sensors, catalytic converter and exhaust system. Everything looked ok. After driving about 40 miles, the P0420 came on again. Did a mega seafoam treatment, reset OBD and 40 miles later, SES and P0420 came on again.

Replaced the oxygen sensors with new Delphi sensors, and the P0420 came on after 80 miles. Tried all kinds of combinations, but nothing worked except my wallet buying fuel, driving around to see if code returned.

Purchased and replaced the catalytic converter. That corrected the P0420 issue, but now I could not get 5 of the 6 required OBDII ready states for emissions testing. I tried driving a few hundred miles, but only 4 of the 6 ready came up. Did some research and discovered the drive cycle to get the ready conditions set.

What I discovered after two weeks was there are two GM drive cycles and that the 2001 Bonneville only likes one of them. The main issue was to get the heated oxygen sensor to ready. What is published most often to get the heated oxygen sensor to cycle to ready is t start engine when cold, and let it idle for 3 minutes with a/c and rear defrost on. This does not work for the Bonny. What is required for the Bonny is to start engine when cold, and immediately put car into drive (with foot on brake) for three minutes.

I tested the above a few times and the results stayed the same. I am able to get 5 of the 6 Bonny tests to ready mode in under ten minutes if starting from a cold engine, which is better than driving hundreds of miles and hoping the tests cycle. The right test cycle procedure was the critical factor.

Have you tried the steps in How to set readiness of emissions?

Yes I did, and discovered on my 2001 SSEi that the below step doesn't trigger the ready.


2. Idle the engine in drive for 2 1/2 minutes with the air conditioner and rear defroster on. You want a lot of electrical load. The more the better. This will run diagnostic tests for O2 heater, Purge "No Flow", Misfire, Passive Air, Fuel Trim and if closed loop is achieved.

What triggers the ready is to start and put into drive immediately (with foot on brake) for 3 minutes. The difference maker was to idle the Bonny in gear, not in park, I did not run have to run the a/c or the rear defrost.

Sad thing is you could have drove up here, paid for a tune and I could have fixed you right up.

Dan,

You are right and after all time and money- I would of done that looking back.....

Hind sight is 20/20

After pulling my hair out for this month to pass smog test here in Vegas, I finally found your advice! It seems all the normal drive cycle information is copies of copies and so on. I've been putting crazy time and gas into the Pontiac Drive Cycle GM has and the freaking 02 sensor and evap never complete and never reset. I will be forever in your debt if this makes my '01 SSEi reset the system so I can get it to pass smog before the end of the month. The penalties are already gonna hurt.....

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