P0411 Headscratcher

New August 7, 2011

What would cause a P0411 code to come on AFTER an extended driving period on the highway? Doesn't come on right away. only after an hour or so on the highway and then the CEL sets.

That should be a start up code, no?

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Eric Lehmann
97 Ebony 42k Welded - With the Angels Now so that I don't have to be. Long Live the Garage Queen !
97 Ebony 198k Resurrected, Welded
96 Medium Willow Green Metallic 150k, blown nomo ;-(
96 Rosemist 74k 2006 Best Of SHO, Popular Vote, 2007 Best Gen 3, Welded
96 Charcoal Grey 152k - Resurrected, Welded
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Eric, I'm guessing that you get the CEL at the point of slowing down to stop or turn. The '96 PCED manual states that the Secondary Air Injection Monitor requires almost 12 minutes of vehicle operating time from initial start-up. When you look at a chart of the drive cycle, you see that the SAI Monitor closes out the drive cycle at a point where the engine comes back to idle while in Drive. The system description states that the functional check may be done in two parts; at startup or during a hot idle if the startup test was not able to be performed. I had this problem a couple years back and had a terrible time tracking down the problem during winter. The frequency of occurrence increased with warming weather. I finally traced the issue to the bypass solenoid which would ohm out OK when cold but would open at some elevated temperature. I found that it was okay during summer morning temperatures but would open later in the day as the garage temperature heated to it's max in the afternoon. Good luck, Lucien

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