P0420 fault code - Catalyst system, bank 1

P0420: Catalytic converter efficiency fault (Renault 1.2 16V engine, 74 hp)

P0420 turns on the check engine light and may indicate a significant fault or excessive emissions. The vehicle's control system has detected reduced catalytic converter efficiency, which can lead to excessive exhaust emissions.

The software window shows the vehicle's exhaust system configuration. In this example, the four-cylinder engine has one exhaust line fitted with two oxygen sensors, also known as lambda sensors. They measure oxygen in the exhaust gases: the upstream sensor supports air-fuel mixture control, while the downstream sensor monitors catalytic converter efficiency.

Oxygen sensor, bank 1, sensor 1: the upstream sensor is mainly used to control how much fuel is injected into the engine.

Oxygen sensor, bank 1, sensor 2: the downstream sensor is mainly used to monitor catalytic converter efficiency.

Confirming the catalytic converter fault

In this case, the engine ECU detected a catalytic converter fault from the downstream oxygen sensor readings. However, do not base the diagnosis on this finding alone. First check the following:

1. A fault in the downstream sensor itself can give a misleading result. Check its connections. As the sensor is often underneath the vehicle, its wiring harness can be damaged. Also check for related fault codes in the Diagnostics tab of EOBD Facile.
2. Damage to the exhaust system can distort downstream sensor readings. An exhaust leak can admit air and affect the oxygen measurement even when the sensor itself is working correctly.
3. An ignition fault causing misfires can damage the catalytic converter prematurely. Check for related engine fault codes, the condition and replacement history of the spark plugs, and any faulty ignition coils (see our article on P0300 misfire fault codes).

If no other fault is found, the catalytic converter itself may be the cause and may need replacing. Catalytic converters have a limited service life, estimated here at approximately 150,000 to 200,000 km.

Replacing oxygen sensors

When replacing the catalytic converter, consider whether the oxygen sensors also need replacing. Their service life is limited and can be similar to that of the converter, estimated here at around 160,000 km.

Use the software to compare the measured sensor values with their typical operating values. Readings persistently close to the upper or lower limit may indicate a worn sensor, but should be assessed as part of the complete diagnosis.

This information is available in EOBD Facile, in either the "Oxygen Sensors" tab or the "Monitor Test Results" tab.