ECM DTC EO43: Left O2 Sensor Rich: Description

The ECM provides a 0.45 volt reference signal to the oxygen sensor on CKT 412. When the oxygen sensor is cold (below 200°C), the oxygen sensor signal voltage will be around 0.45 volts and the ECM will keep the system is open loop operation. When the oxygen sensor is warm (above 200°C), the oxygen sensor will swing from rich to lean rapidly, at least one swing every two seconds, if the ECM is in good control of the air fuel mixture.

When the ECM sees that the oxygen is varying from the cold voltage of 0.45 volt, it will send the system into closed loop operation. In closed loop operation, the ECM will adjust the fuel delivery rate to the engine based on the oxygen sensor readings. Code EO43 is designed so that if the oxygen sensor stays at a rich voltage for more than 50 seconds during the test conditions, Code EO43 will set. Code EO43 will set when there is an oxygen sensor circuit fault giving a false rich indication or when the air fuel ratio is actually rich due to a fuel control or emission system fault.