Warm Up Three-Way Catalytic converter deterioration or malfunction
Exhaust system leaks
Loose front or rear HO2Ss
Faulty front heater O2 sensor
O2 sensors harness is open or shorted
O2 sensors poor electrical connection
Engine Light ON (or Service Engine Soon Warning Light)
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. The P0431 code is set when the Engine Control Module (ECM) detects the rear HO2S value difference becomes closer to the value of the front HO2S. The ECM monitors when the vehicle speed is 25-60 MPH, the load is 22-45 percent, and the engine speed is 1000-3000 RPM.