I put this in the Superduty/Heavyduty forum and received a response to check for vacuum leaks. I will try that, but the thing is that I only get the P1131 (OBD II) under high vacuum during heavy acceleration (probably won't see the vacuum leak during testing in my driveway).

I have a 1997 F350 460 dually crew cab and haul a Lance camper in the bed. I don't drive it much - typically only on vacations. During my last three outings the CEL has come on with the same code (P1131). The CEL only comes on after several hours of driving and during periods of heavy acceleration (long inclines). This is the code for the upstream O2 sensor on the passenger side indicating a lean condition.

I have a Scanguage II and have been able to clear the code after a while, so I know the lean condition is temporary. I have cleaned the MAF twice thinking that may be the problem. I don't hear any exhaust leaks or see any evidence of leaks at the intake.

Before I replace the O2 sensor, is there anything that may cause this intermittent problem? It doesn't show up on short drives around town. I am thinking that it may be related to the elevation that the truck is at when the CEL comes on (less oxygen at higher altitudes).

Thanks