For 1990-2009 cars

### Circuit Description

Catalytic converter efficiency is measured by how well the converter can store and release oxygen needed to convert CO, HC, and NOx gases to CO2 and water. Under normal operating conditions except heavy acceleration, cold engine temperatures and extended decelerations, the heated oxygen sensor (HO2S) bank 2 sensor 2 voltage should remain relatively steady between 500-800 mV. This steady reading of the HO2S bank 2 sensor 2 voltage indicates a correctly functioning pre-catalytic converter as the air/fuel ratio changes slightly around 14.7:1. If the pre-converter is storing and releasing oxygen correctly, the HO2S bank 2 sensor 2 should hold its voltage value for a certain length of time as the air/fuel ratio increases or decreases. Under relatively steady cruise, the engine control module (ECM) will run a test which records the voltage amplitude values of the HO2S bank 2 sensor 2 as an oscillating air/fuel ratio is introduced. The ECM will then compare the amplitudes it obtained to modeled values of a near-failed catalyst. The closer the actual amplitudes of the HO2S bank 2 sensor 2 are to the modeled near-failed catalyst values, the more inefficient the pre-converter is becoming. DTC P0431 sets when the ECM has determined that the bank 2 pre-catalytic converter is no longer efficient.

### Conditions for Setting the DTC

DTC P0431 will set if the ECM has determined the bank 2 pre-catalytic converter to be inefficient by using the HO2S bank 2 sensor 2 when:

• | The engine speed is between 1000-2120 RPM.

• | The loop status bank 2 is closed.

• | The engine load is between 20-45 percent.

• | The calculated converted temperature is greater than 350°C (662°F).

• | No evaporative emission (EVAP) purge solenoid, EVAP flow during non-purge, fuel trim bank 2, HO2S bank 2 sensor 1, HO2S bank 2 sensor 2, mass air flow MAF, misfire, throttle position (TP) sensor 1, or TP sensor 2 DTCs have been set.

• | DTC P0431 diagnostic is run once for 50 seconds once the above conditions have been met

DTC P0431 is a type B DTC.

### Diagnostic Aids

Make sure no exhaust leaks exist between the HO2S bank 2 sensor 1 and 18 in. after the HO2S bank 2 sensor 2. An exhaust leak in that area will cause false HO2S bank 2 sensor 2 readings and may set DTC P0431. Repair a leak if one exists and drive the vehicle to verify the DTC P0431 diagnosis has run and passed. Follow the enable conditions under DTC parameters.

A misfiring cylinder on bank 2 pushing raw fuel out the exhaust port will damage the pre-converter over time. Verify that no current or history misfire counts exist in any failure record. Repair a currently misfiring cylinder if one exists.

Replace the bank 2 pre-catalytic converter.

Bank 2, cylinders 2, 4, 6, are closest to the front of the vehicle.