CODE 44,45,55/CHECK ENGINE OXYGEN SENSOR TEST PROCEDURE

Many oxygen sensors are being replaced to resolve incidents of "check engine" lights being set or Codes 44, 45, and 55 being stored. The oxygen sensor is not always the cause, and a simple on-car check can be performed to determine if the sensor is at fault.

1. The engine should be run for three (3) minutes on the high step of the fast idle cam with the M/C solenoid dwell lead grounded. This will burn off any carbon that might affect oxygen sensor response to exhaust gas.

2. Next remove the ground from dwell lead and connect dwell meter. The engine should now be at idle. While observing dwell meter, choke the engine, the dwell should begin to rise to 550 within two (2) seconds of choking. (If dwell is already at 550 follow next step only.)

3. Disconnect vacuum hose at power brake booster and dwell should begin to drop to 60 within two (2) seconds.

If the oxygen sensor responds correctly to the above procedure, it is not defective and should not be replaced. The time for the dwell to actually reach 550 or 60 will vary with engine.

By connecting digital voltmeter J-29125 to ECM terminals nine (9) and fourteen (14) of the black connector, the oxygen sensor voltage may be monitored. In a normal system, the voltage will swing back and forth from a low value (100-300mv) to a high value (700-900mv). If the exhaust gas is rich, the voltage will stay high. If exhaust is lean, voltage will stay low. A lean exhaust, low 02 sensor voltage will cause a rich command, low dwell reading. A rich exhaust, high 02 sensor voltage causes a lean command, high dwell reading.

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.