My dad has a 1998 Honda civic lx sedan 1.6 automatic with 63,000 miles. Recently the check engine light came on. The car runs well, gets great gas mileage and I can detect no engine missing or other problems. Spark plugs, timing belt, oil & filter were changed about 2,000 miles ago along with a valve adjustment. I purchased a OBD 2 code reader and found it was code, PO 135, the O2 sensor heater circuit. I replaced the sensor with a new one and the light went out after resetting. Now, after driving for about 500 miles without the check engine light on, it is back on. The car has all the power it ever had and no other noticeable symptoms. I think something else is wrong and I probably wasted time and money on the O2 sensor, but the code reader isn't giving me a second choice! Any ideas? Thank you.