Oxygen Sensor Heater
- Unplug O2 sensor connector. Using ohmmeter, measure resistance between sensor terminals. Replace sensor if resistance is not within 5-6 ohms at 68°F (20°C). See Fig 1.
- A trouble code No. 21 is also an indicator of a problem in O2 sensor circuit. Trouble code No. 21 will set if O2 sensor heater circuit opens or shorts to ground on "HT" wire from sensor to ECU.