Chart A-6 Open Loop Diagnosis

Chart A-6:

DFI Oxygen Sensor Circuit:

When the oxygen sensor is cold (below 200°C), the output voltage will be around 0.5 volts and the ECM will keep the system in open loop operation. When the oxygen sensor is warm (above 200°C), the output voltage will swing between 0.0 and 1.0 volts. When the ECM sees that the oxygen sensor is not at a cold voltage of 0.5 volt and the coolant sensor value is high enough, it will send the system into closed loop operation. In closed loop operation, the ECM will meter fuel into the engine based on the oxygen sensor readings. This procedure should only be used when the vehicle will not go into closed loop operation.

1. To check the operation of the oxygen sensor, observe engine data parameter P.O.7 with the oxygen sensor disconnected. If the oxygen sensor voltage is less than 0.37 or greater than 0.57, the wiring harness must be investigated for defects. If the wiring is OK, then the ECM connector and ECM must be checked because the system is not functioning properly.

2. If the voltage is between 0.37 and 0.57 volts, then jumper the oxygen sensor harness pin (ECM side) to ground and note the voltage reading. If the voltage reading is greater than 0.05 volts, then the ECM is not able to recognize that a fault was artificially created. Therefore, either the ECM is not functioning properly or an oxygen sensor circuit is open between the connector and the ECM.

3. If the voltage reading is less than 0.05 volts, then the ECM is functioning properly and the harness is OK. The oxygen sensor should be checked using the procedure on Chart A-9 - Oxygen Sensor Diagnosis.

4. The ECM compares oxygen sensor signal voltage received on circuit 412 to the ground voltage on circuit 413. If the ECM doesn't have a good ground to the engine on circuit 413, the oxygen sensor can appear falsely high or low. With engine running, use a voltmeter to measure voltage from the oxygen sensor at the exhaust manifold to the ground eyelet for the oxygen sensor (0.8 mm tan wire) at the generator. If the voltage is -0.05 volts to +0.05 volts then the ground is OK. If the voltage is less than -0.05 volts or greater than +0.05 volts, tighten the ground stud at the generator (torque to 28-37 Nm).