P0439 Subaru Catalyst Heater Control Circuit Bank 2

Possible causes

– Catalyst Temperature Sensor harness is open or shorted
– Catalyst Temperature Sensor circuit poor electrical connection
– Faulty Catalyst Temperature Sensor

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0439 Subaru Description

The Engine Control Module (ECM) monitors the switching frequency ratio of heated oxygen sensors 1 (front O2 sensor) and heated oxygen sensors 2 (rear O2 sensor).

A three way catalyst converter (Manifold) with high oxygen storage capacity will indicate a low switching frequency of heated oxygen sensor 2. As oxygen storage capacity decreases, the heated oxygen sensor 2 switching frequency will increase.

When the frequency ratio of heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst malfunction is diagnosed.