Checking Oxygen Sensor Heater

SYMPTOM
* CHECKING O2S HEATER

POSSIBLE CAUSES
- Downstream O2 sensor defective
- Upstream O2 sensor defective
- ASD rly out ckt open (downstream O2 sen disconn)
- ASD rly output ckt open (upstream O2 sen disconn)
- Heater ground ckt open (dwnstrm O2 sen disconn)
- Heater ground ckt open (upstream O2 sen disconn)