For 1990-2009 cars

Three-way catalytic converter (TWC) efficiency is measured by how well the catalytic convertor can store oxygen. The powertrain control module (PCM) monitors converter efficiency by comparing the voltage values of the oxygen sensor 1 (O2S-1) and heated oxygen sensor 2 (HO2S-2). Under normal operating conditions, the O2S-1 should vary between 10-999 mV and the HO2S-2 should remain relatively steady between 500-800 mV. This steady reading of the HO2S-2 indicates a correctly functioning catalytic converter. When all parameters have been met, the PCM will run DTC P0420 diagnostic at idle. The PCM will command rich and monitor the time it takes the HO2S-2 to go rich. It will then command lean and monitor the time it takes the HO2S-2 to go lean. The longer it takes the HO2S-2 to change rich/lean means the converter is storing oxygen and is functioning properly. The PCM will usually run a 5 second test to determine the efficiency of the converter. If this test fails, the PCM may take several tests during several ignition cycles to set the DTC. DTC P0420 sets when the PCM has determined that the catalytic converter is no longer efficient.

### DTC Parameters

DTC P0420 will set if the PCM has determined the catalytic converter to be inefficient by using the O2S-1 and HO2S-2 when:

• | The engine run time is greater than 9.5 min.

• | The engine speed is between 650-900 RPM.

• | The engine speed over 1,200 RPM for 30 seconds after achieving Closed Loop.

• | The STFT is between 122-134 counts.

• | The calculated converter temp is between 375-750°C (707-1,382°F).

• | The vehicle speed equals 0 mph.

• | The Loop status is closed.

• | The manifold absolute pressure (MAP) change is less than 6 kPa.

• | No camshaft position (CMP), crankshaft position (CKP), engine coolant temperature (ECT), exhaust gas recirculation (EGR), evaporative emission (EVAP) purge solenoid, 5-volt reference, fuel trim, HO2S-2, intake air temperature (IAT), idle speed, MAP, misfire, O2S-1 or TP or vehicle speed DTCs have been set

DTC P0420 diagnostic is sampled once at idle once the above conditions have been met. If that test fails, the PCM will take more samples during the current ignition cycle or may take more samples over several ignition cycles. The extra sampling is to ensure that a converter has truly failed.

DTC P0420 is a type A DTC.

### Diagnostic Aids

The cause of DTC P0420 is an inefficient catalytic converter that can no longer perform its function to store oxygen.

A misfiring cylinder pushing raw fuel out the exhaust port will damage the 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 underfloor catalytic converter.