For 1990-2009 cars

Three-way catalytic converter (TWC) efficiency is measured by how well it can store oxygen. The PCM monitors converter efficiency by comparing the voltage values of the heated oxygen sensor 1 (HO2S-1) and heated oxygen sensor 2 (HO2S-2). Under normal operating conditions, the HO2S-1 should vary between 10-1065 mV and the H02S-2 should remain relatively steady between 500-800 mV. This steady reading of the H02S-2 indicates a correctly functioning catalytic converter. When all parameters have been met, the PCM will run a 5 second 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 H02S-2 to go lean. The longer it takes the H02S-2 to change rich/lean means the converter is storing oxygen and is functioning properly. If the 5 second 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 HO2S-1 and HO2S-2 when:

• | The engine run time is greater than 8 minutes and 50 seconds.

• | The engine speed is at idle within -75 to +150 RPM of the desired idle speed.

• | The engine speed is over 1000 RPM for 35 seconds (auto), or 45 seconds (manual) after achieving closed loop.

• | The engine has not been at idle for more than 47 seconds. Once off idle, the counter resets.

• | The ECT is between 70-125°C (158-257°F).

• | The IAT is between -21°C and +80°C (-6°F and +176°F).

• | The calculated converter temp is between 501-627°C (933-1161°F).

• | The vehicle speed is less than 5 km/h (3 mph).

• | The calculated air flow is less than 8 g/s.

• | The ignition voltage is greater than 9 volts.

• | The barometric pressure is greater than 72 kPa.

• | No cam, CKP, ECT, EVAP, fuel tank pressure, FC relay, fuel injector, fuel trim, HO2S-2, IAT, idle speed, knock, MAP, misfire, HO2S-1, PCM internal fault, 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 up to 6 ignition cycles. The extra sampling is to ensure that a converter has truly failed.

DTC P0420 is a type A DTC.

### Diagnostic Aids

Important: Make sure the HO2S-2 is tight in the exhaust pipe. A ground path must exist between the exhaust pipe and engine block. Add a ground wire between the exhaust manifold and engine head if resistance is greater then 10 ohms. DTC P0420 may set if no ground path exists.

Make sure no exhaust leaks exist between the HO2S-1 and 18 inches after the HO2S-2. An exhaust leak in that area will cause false HO2S-2 readings and may set DTC P0420. Repair a leak if one exists.

Misfiring cylinders 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 any currently misfiring cylinders if any exists.

Replace the underfloor catalytic converter.