probably the O2 sensor and could be intermittent - what code is showing on the ECU? pull back the carpet on the passenger side and look at the top of the ECU - with ignition switch on, you should see a flashing red LED - count the number of times it flashes between pauses and that will give you the code - O2 sensor is code 1 (flashes once, pauses, flashes once, etc.) - to reset the ECU to clear the codes, you need to remove the hazard fuse from the main underhood fuse box for about 20 seconds and then replace the fuse - once you have a check engine light showing on the dash, the ECU will store the codes where they will remain until you manually clear them - codes will show on the ECU even if the check engine light on the dash is off