My four banger 95 Contour GL developed a new problem... when idling (cold or hot) the engine revs up periodically. About every two seconds the RPMs go up momentarily and then back to normal. CEL did not come on but I checked the KOEO error codes anyway. I got 335 (exhaust gas diff sensor) followed by 539 (A/C on, rerun test). The second code is bogus - the A/C was not on during the test. I reset the computer, restarted the engine and then checked the codes again. They were the same. Any suggestions?
The car has ~100k on it, automatic, the exhaus gas differential sensor WAS replaced in 11/2003. Could it go bad that soon?
