Diagnostic Aids

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 O and 1.0 volts. When the ECM sees that the oxygen sensor is not at a cold voltage of 0.5 volt, 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.

1. With the engine running warm (coolant temperature 85°C or greater) and at fast idle, observe engine data parameter .0.7. If there is no voltage variation and if the ECM and harness are operating properly, check for a faulty oxygen sensor connection or a faulty oxygen sensor. If the voltage remains between 30 and 60 for a minimum of one minute, check for the possibility of leaded or contaminated fuel (i.e., filler neck or catalytic converter has been tampered with) before checking for a faulty oxygen sensor connection or a faulty oxygen sensor.
2. While running the engine at a fast idle, observe the "open/closed loop" status light above the "off" button on the ECC control head. If the ECM switches the system to closed loop operation, then the oxygen sensor is OK If the ECM remains in open loop operation, then replace the oxygen sensor.