P0420 code after coil change

P0420 code after coil change

Postby sieng123 [OP] » January 12th 2020, 2:57am

I have a 1999 safari. It started running rough in wet and damp weather and then finally wouldn't start at all. Took the dog house off and cranked it over and could see blue flashes coming from the coil. I replaced the coil with a new AC delco and it fired right up, runs smoother than ever. However, about 5 miles down the road I got a check engine light, scanned it and turned out to be a P0420. Never had a check engine light in the 170,000 miles on it before. Don't have a clue what the issue could be. I have never changed the cat, exhaust, or any O2 sensors, its all original. A strange thing about it, after resetting the CEL, it will come back on in warmer weather sooner than colder weather. When the temp is around 32 F (0 C) the CEL will come on in a couple days, but when the temp is about 5 F (-15 C) it will take a week to ten days until it comes back on. Anyone have any ideas?? Thanks for the help!!!!
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Re: P0420 code after coil change

Postby Leeann_93 » January 12th 2020, 3:22am

PO420 can mean back upstream oxygen sensors, but it also can mean that the cat is not operating efficiently. On my white van, that meant the cat was no longer efficient because it was damaged by the misfires caused by the lack of tuneups by the PO. It was free-flowing, but not working. On my ‘05 van, the cat was irreparably damaged by the leaking injectors and ignition “tuning” meant to cover for the failing injectors, so it was plugged - almost completely.

The leaking spark on the coil means that the plugs weren’t firing properly and unburned fuel was dumped into the exhaust for a while, and unburned fuel will melt a catalyst pretty quickly.

First thing to do is to replace both upstream and downstream oxygen sensors, and not with Bosch sensors. Get Delphi or NTK sensors only. If the code persists or comes back, you probably are going to have to replace the y-pipe.

You’ve hopefully fixed the ignition problem that caused the cat to fail, so there should be no issues once you replace the cat.

Warmer weather lets the vehicle go into closed loop more quickly because the van warms up to temp more quickly. Sensors don’t report until the van is up to operating temp.
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Re: P0420 code after coil change

Postby AstroWill » January 12th 2020, 5:24pm

sieng123 wrote:I have a 1999 safari. It started running rough in wet and damp weather and then finally wouldn't start at all. Took the dog house off and cranked it over and could see blue flashes coming from the coil. I replaced the coil with a new AC delco and it fired right up, runs smoother than ever.

What usually kills the ignition coil is a failing secondary ignition system. I would check your cap/rotor/wires/plugs and also check for Pancake Distributor Disease while you are in there.


sieng123 wrote:However, about 5 miles down the road I got a check engine light, scanned it and turned out to be a P0420. Never had a check engine light in the 170,000 miles on it before. Don't have a clue what the issue could be. I have never changed the cat, exhaust, or any O2 sensors, its all original. A strange thing about it, after resetting the CEL, it will come back on in warmer weather sooner than colder weather. When the temp is around 32 F (0 C) the CEL will come on in a couple days, but when the temp is about 5 F (-15 C) it will take a week to ten days until it comes back on. Anyone have any ideas?? Thanks for the help!!!!

The difference in time might be because everything has to reach a certain state before the PCM decides that it can look at the rear sensor to see if the cat is working. Think temperature/closed loop for x amount of time.
I would check what I mentioned above, and then erase the code and see if it returns.

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