Inspection Procedure

DTC 21 Main Heated Oxygen Sensor Circuit

INSPECTION PROCEDURE
1. CHECK VOLTAGE BETWEEN TERMINAL HT1 OF ECM CONNECTOR AND BODY GROUND.
(a) Connect the check Harness A.



(b) Measure voltage between terminal HT1 of ECM connector and body ground.

OK if:.... 9.0 - 14.0 volts

- If OK proceed to step 3.
- If not OK proceed to step 2.

2. CHECK MAIN HEATED OXYGEN SENSOR HEATER.
(a) Disconnect main heated oxygen sensor connector.



(b) Measure resistance between terminals 1 and 2 of main heated oxygen sensor connector.

OK if:... Resistance: 11.0 - 16.0 Ohms
{at 20°C (68°F)}

- If OK check and repair harness or connector between main relay and main heated oxygen sensor, main heated oxygen sensor and engine control module.
- If not OK replace main heated oxygen sensor.

3. CHECK VOLTAGE BETWEEN TERMINALS HT1 OF ENGINE CONTROL MODULE CONNECTOR AND BODY GROUND.
(a) Warm up engine to normal operating temperature.



(b) Measure voltage between terminals HT1 of engine control module connector and body ground, when engine is idling and racing at 4,000 rpm.

Hint: In the 4,000 rpm racing check, continue engine racing at 4,000 rpm for approx. 20 seconds or more.

OK if:
Voltage Idling... 0.0 volts
Voltage @ 4,000 rpm... 9.0 - 14.0 volts


- If OK replace main heated oxygen sensor [1].
- If not OK check and replace ECM.

[1] It is probable the oxygen sensor has deteriorated. Usually, this can not be confirmed by visual inspection.

INSPECTION USING OSCILLOSCOPE