About 3 mo. ago the "Check Engine" illuminated on morning start. There was no perceptible change in engine performance so I drove it several days and then took it to a Volvo mechanic who read the codes ECM 261 and ECM 271. He said this signaled a vacuum leak, which he could not find. The computer was reset. No light for several days and then it reappeared. I returned and the mechanic did a "smoke test" but did not detect a leak. He checked O2 performance scope and opined that back sensors might be malfunctioning so they were replaced. Computer was reset. Light stayed off several days and then back on. I replaced fuel cap. Light stayed on, then after several days it was off. Then back on. Found another Volvo mechanic. He checked it, found nothing, reset. Light off a couple of days and then on. He replaced fuel filter and put injector cleaner in the system. Said I might need another smoke test. There is no perceptible change in engine performance with light on or off. I'm thinking its a stray signal that comes on when computer does whatever it does on start up. Anybody else have this problem with an S80

 

The evap valve might be bad.

 

Thanks for the tip. I'll check with my mechanic. I'll post the results.