I have this ongoing issue with the "check engine" light in the instrument cluster coming on at partial throttle. Tromp on the gas pedal, the light goes out. Release the gas pedal, the light goes out. It only comes on at partial throttle. Code (13) indicates O2 sensor, but there are no symptoms that would indicate it is malfunctioning. It runs fine, accelerates fine, no hiccups and the sensor's less than a year old. The handy Haynes manual has that asterix that says Code 13 may or may not be the O2 sensor, but no pointers as what the problem might be if the sensor is good. Is there any way to test the O2 sensor short of buying a new one. And what else could cause a Code 13?