Code P0420: catalytic converter efficiency below threshold (bank 1)

Original definition (SAE J2012): Catalyst System Efficiency Below Threshold (Bank 1)^[1][2]

• P0420Catalyst system efficiency below threshold (bank 1).
• P0430Catalyst system efficiency below threshold (bank 2). It is the same fault as P0420, but relating to the bank 2 catalytic converter, which only exists on engines with two cylinder banks (for example, V6 or V8).^[3][6][7]
• P0421Warm-up catalyst efficiency below threshold (bank 1). It is a code similar to P0420, which refers to the catalyst system as a whole, but relating to the warm-up catalytic converter, the one closest to the engine: during warm-up after a cold start, the ECU checks that it quickly reaches the required efficiency.^{[8][9][10][11]}

1. 1Worn or damaged catalytic converter that no longer stores and releases oxygen properly. It is the most common cause, especially on high-mileage cars.^[2][3][4]
2. 2Faulty or aged lambda sensor, or one with a poor electrical connection. The rear one can give a wrong signal that mimics a bad catalytic converter; if the front one fails, the mixture goes out of control and can end up damaging the catalytic converter.^[1][2][3][4]
3. 3Exhaust leaks before the catalytic converter or between the sensors (manifold, pipes or the catalytic converter itself), which distort the oxygen measurement.^[1][2][3]
4. 4An underlying problem that punishes the catalytic converter: misfires, a leaking injector, a mixture that is too rich or too lean, an engine burning oil, or coolant entering the cylinders through the head gasket. The catalytic converter is then the victim and, if the cause is not fixed, the new one will be ruined just the same.^[1][2][3][4]
5. 5Contaminated catalytic converter, for example by high-sulphur fuel or by oil.^[1][2][3]
6. 6Unsuitable replacement catalytic converter: one of lower quality than the original, or fitted too far from the engine, may not reach working temperature and make the code come back.^[2][4]
7. 7Out-of-date ECU software: some manufacturers fix the code with an update instead of replacing the catalytic converter.^[3]

• Engine warning light on. In many cases it is the only symptom and the car runs normally.^[1][2][3][4]Engine warning light: faults that cause it
• Sometimes, a slight loss of power.^[1]Less power: faults that cause it
• Slightly higher fuel consumption.^[2][4]Uses more fuel: faults that cause it
• Smell of sulphur (rotten eggs) from the exhaust on some cars, or of unburnt fuel.^[2][3][4]
• If the ceramic core of the catalytic converter has broken up, a metallic rattle under the car; if it has burnt out, the exhaust sounds louder than normal.^[3]

1. 1Read all the stored codes and the freeze-frame data with a diagnostic tool. If there are misfire, mixture or sensor codes, resolve them first and check again: they may be the real cause of the P0420.^[2][4]
2. 2Look for technical service bulletins or manufacturer campaigns for your model: in some cases they recommend a simpler fix than replacing the catalytic converter, such as updating the ECU software.^[3]
3. 3Check the exhaust from the manifold to the sensors for leaks and repair them before going on with the tests, because they distort the readings.^[1][3]
4. 4Check the connectors and wiring of the lambda sensors and compare their live signals: the front one should oscillate and the rear one should stay steady. If the rear one copies the front one, the catalytic converter is not working properly; if there is any doubt about a sensor, replace it before the catalytic converter, which is a much more expensive repair.^[1][3][4]
5. 5Clear the code and take a test drive with the engine warm at a steady speed for 10-20 minutes; then use the diagnostic tool to check whether the catalyst monitor completes and whether the code returns. If it does not come back, nothing needs repairing for now.^[3]
6. 6Use an infrared thermometer to measure the pipe temperature before and after the catalytic converter: if it is working, the outlet pipe is hotter than the inlet one.^[3]
7. 7If the catalytic converter is confirmed as degraded, replace it with one to original specification and first find out what damaged it (oil, misfires, injector, coolant) so that the new one does not fail in the same way.^[2][3][4]

• Injector

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• Ignition coil

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Faults in which this code appears

• Medium severityFrequency: Common1.3, 1.5 and 1.6 gasolina (serie M)Faulty catalytic converter and lambda sensors on the 2005-2010 Swift (engine warning light)On the 2005-2010 Swift the catalytic converter often fails at around 50,000 km according to gebrauchtwagenberater.de, and several 1.6 owners cite the lambda sensor. The warning is the engine light; on the Sport the catalytic converter is integrated with the manifold.
• Low severityFrequency: Occasional1.0 VVT-iFaulty lambda sensor (P0420 code) on the 1.0 VVT-iOn the 1.0 VVT-i (1KR-FE) in the Aygo, C1 and 107/108, the lambda sensor, especially the one upstream of the catalytic converter, is documented as failing at high mileage and leaving code P0420 and the engine warning light. It is a cheap repair and does not damage the engine.

• P0420 on BMW12
• P0420 on Seat12
• P0420 on Škoda12
• P0420 on Volkswagen12
• P0420 on Citroën10
• P0420 on Peugeot10
• P0420 on Renault9
• P0420 on Audi7
• P0420 on Dacia7
• P0420 on Fiat7
• P0420 on Toyota7
• P0420 on Mazda6
• P0420 on Nissan6
• P0420 on Opel6
• P0420 on DS5
• P0420 on Ford5
• P0420 on Lancia5
• P0420 on Mercedes-Benz5
• P0420 on Jeep4
• P0420 on MINI4