I have a 1999 civic 4 door LX (from Canada). My check engine came on and code was 1 (primary oxygen sensor?). I checked both the one before and after the cat converter and the signal seemed good (about 0.9v with engine hot. But the heater voltage was missing on the lower sensor (after the cat converter) with the ignition ON. this was measured with the sensor unplugged and the voltmeter on the car wiring harness. I measured 12V on upper heater in same condition. Heater resistance seemed normal on both (about 15 ohms). Do I have a wiring problem or is the fault code causing the ECU to shut off heater voltage on this sensor? Also, which is considered primary and secondary o2 sensor? Tracing the wire in the engine harness is almost impossible.