C 1500 Truck 2WD V6-262 4.3L VIN Z (1992)
Oxygen Sensor: Technical Service Bulletins
Engine Controls - Heated Oxygen Sensor Diagnosis
Group Ref.: Engine Fuel & Emission
Bulletin No.: 366501
Date: November, 1993
INFORMATION
SUBJECT:
HEATED OXYGEN SENSOR DIAGNOSIS
MODELS:
1992-93 CHEVROLET & GMC LIGHT DUTY TRUCKS WITH 4.3L (LB4/L35), 5.0L (L03) AND 5.7L (L05) ENGINES
1992-93 OLDSMOBILE BRAVADA
Some heated oxygen sensors are being replaced in error due to low reference voltage.
REFERENCING SERVICE MANUAL
Before diagnosing any heated oxygen sensor, be sure to follow the appropriate service manual diagnostics. The fault tree selected is dependent upon
which control module system is on the vehicle.
DIAGNOSTIC INFORMATION
The electronic control module (ECM) normally applies a reference voltage of approximately 450mv to the oxygen sensor. When the ignition is turned
"on", the heating element inside the oxygen sensor will immediately start heating the sensor element. The oxygen sensor will become fully functional in
15 to 90 seconds. Because the engine is not running, the gases around the oxygen sensor in the exhaust stream will be mostly ambient air.
The high amount of oxygen in the exhaust stream will be interpreted as a lean exhaust and the 450mv reference voltage from the ECM will be pulled
low. The oxygen sensor voltage should drop from 450mv to less than 100mv within 90 seconds. Therefore, the oxygen sensor is performing normally.
LOW REFERENCE VOLTAGE (This test must be completed within 10 seconds after key up.)
If your Tech 1 initial reference voltage is low (less than 300mv), you may have a shorted oxygen sensor. Turn the ignition "off"; allow the sensor to cool
for ten minutes and retest. If the voltage is still low, disconnect the oxygen sensor and see if your Tech I now indicates approximately 450mv reference
voltage. If so, change the oxygen sensor.
HIGH REFERENCE VOLTAGE
If your Tech 1 initial reference voltage is high (greater than 600mv), you may have a sensor signal that is shorted to heater voltage. This may cause a
code 45. Allow sensor to cool ten minutes and retest. If voltage still remains high, disconnect oxygen sensor. If reference voltage still remains high, you
may have a short to voltage in the engine harness or an ECM problem.
If you disconnect the oxygen sensor and reference voltage returns to approximately 450mv, replace the internally shorted oxygen sensor.
