INFORMATION ON HEATED OXYGEN SENSORS DIAGNOSTICS. GMC Truck S15 Pickup

SUBJECT: INFORMATION ON HEATED OXYGEN SENSOR DIAGNOSIS

MODELS: 1992-93 LIGHT DUTY TRUCKS WITH 4.3L (LB4/L35), 5.0L (L03) AND 5.7L (L05) ENGINES

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.

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GENERAL MOTORS BULLETINS ARE INTENDED FOR USE BY PROFESSIONAL TECHNICIANS, NOT A "DO-IT-YOURSELFER". THEY ARE WRITTEN TO INFORM THOSE TECHNICIANS OF CONDITIONS THAT MAY OCCUR ON SOME VEHICLES, OR TO PROVIDE INFORMATION THAT COULD ASSIST IN THE PROPER SERVICE OF A VEHICLE. PROPERLY TRAINED TECHNICIANS HAVE THE EQUIPMENT, TOOLS, SAFETY INSTRUCTIONS AND KNOW-HOW TO DO A JOB PROPERLY AND SAFELY. IF A CONDITION IS DESCRIBED, DO NOT ASSUME THAT THE BULLETIN APPLIES TO YOUR VEHICLE, OR THAT YOUR VEHICLE WILL HAVE THAT CONDITION. SEE A GENERAL MOTORS DEALER SERVICING YOUR BRAND OF GENERAL MOTORS VEHICLE FOR INFORMATION ON WHETHER YOUR VEHICLE MAY BENEFIT FROM THE INFORMATION.