One-time P0420 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 10-04-2010, 09:43 PM   #1
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Cam install, one week later: while driving in the rain today, I got a check engine light while cruising gently uphill: P0420 set, another P0420 pending. I cleared the codes to see if they would return, and they didn't. The code means that I either have a massive exhaust leak, my rear O2 sensors are showing too little change, or my cat is dying.

(Less than a month before my emissions test, and I get my very first emissions-related CEL. It figures.)

Here'* what I've looked at and what I plan to do:
  • I've noticed no loss of top speed or power, so I'm guessing my cat is fine. Last time I gave it a good knocking (during the cam install), nothing rattled.
  • I reused the exhaust pipe gasket when reinstalling the engine, because it was in good shape and not flattened. I'll replace that the next chance I get.
  • I checked the fuel trims and the voltages of the O2 sensors at idle and while cruising (25-35 mph), but that was pointless, seeing as how I don't know how to interpret the data.

Anything else I should check out?
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Old 10-04-2010, 10:02 PM   #2
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Sure..first thing is list the code description.
Ok, I'll do it, but keep in mind it'* your responsibility.

Catalyst below efficiency. This means the difference between the reading on the front and rear O2'* doesn't have enough of a difference. Aka your whacking could have broken up the honeycomb and screwed up your cat.

Stop on over and that code will .... no longer be a problem

ps..bring money to take me to dinner.
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Old 10-05-2010, 11:17 AM   #3
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By "a good knocking," I meant a few taps with my bare knuckles. If that'* enough to do it, find some Kryptonite.

Grabbed my scanner and went out to the highway last night. Did a couple WOT pulls while watching the intake manifold pressure, which fluctuated as it normally should. That rules out a blockage. I'm going to follow the GM eSI P0420 troubleshooting today to see if the cat is flowing but dead.

So, if the code comes back, I'll pay you a dinner visit.
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Old 10-05-2010, 11:52 AM   #4
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Wednesday I'm helping Max. So you'd be buying for both of us. Thurs/Friday I'll be unavailable.
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Old 10-05-2010, 01:26 PM   #5
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Check to see in the hussel and bussel of moving everything around that you didn't crush, expose, or cut any wires.
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