DTC EFI-521

5-2-1 FRONT HEATED OXYGEN SENSOR, HEATING
5-2-2 REAR HEATED OXYGEN SENSOR, HEATING





Trouble Code Conditions
- Monitoring of the heated oxygen sensor (HO2S) heating circuit begins approx. 3 minutes after the HO2S heating sequence starts. Diagnostic trouble code (DTC) EFI-521 or EFI-522 is stored if the engine control module (ECM) registers too high or low a current in the heating circuit.

Substitute value(s)
- Twin heated oxygen sensor fuel trim is disabled.

Possible source(s)
Signal too high:
- Short-circuit to supply voltage in signal cable.
- Defective heated oxygen sensor.

Signal too low:
- Short-circuit to ground in signal cable.
- Open-circuit in signal or power cable.
- Contact resistance in terminals.
- Defective heated oxygen sensor.

Fault symptom(s)
- None.