Fault code P0420 – a step-by-step guide to diagnosing and fixing it

The engine control unit (ECU) triggers this DTC when it detects that the catalytic converter is not working efficiently. The neutralisation component is monitored by two sensors. If the upstream and downstream sensors are showing the similar readings, it could indicate that the oxygen levels are too high.

• Damage or leaks in the exhaust manifold
• Engine misfires
• A leaky fuel injector
• Oxygen and coolant temperature sensor malfunctions
• Excessive fuel pressure
• Oil in the catalytic converter
• Broken wires and faulty electrical connectors
• Open or short circuits
• Exhaust pipe damage or leaks
• A defective muffler

Here are the most common symptoms of the mechanical and electrical problems related to this DTC:

• A “rotten egg” smell coming from the exhaust system. High oxygen levels may lead to an increase in sulphur in the fuel system, producing a foul odour
• The check engine light comes on. This could occur due to a number of reasons. It is important to get a proper diagnosis in order to rectify the issue.
• Slow acceleration and a loss of engine power. You may notice that the vehicle is unable to reach higher speeds than 30-40 mph.
• Higher fuel consumption
• Rough idling
• Starting difficulties

It highly recommended that you conduct a series of diagnostic checks to determine and address the root of the problem. Catalytic converters are very expensive to replace, but the unit can often be restored by simply replacing devices, cables and pipes or decontaminating the unit.

• The first thing you should do is diagnose the fault. You can do this by scanning the system using an OBD2 scan tool and checking the error codes. There is plenty of information available on these errors online.
• Check whether there is a problem with the oxygen sensors themselves. Inspect the components for signs of damage or wear. You should check the voltage in the devices using a voltmeter and compare the readings to the standard values.
• Check the cables and connectors for damage and replace them if necessary. If the monitoring devices are still malfunctioning after new parts are installed, you may need to replace them. It may also be possible to restore the power supply by cleaning the connectors with a specialised cleaning agent for improved conductivity. It will also help to prevent issues such as corrosion in the future.
• Repair or replace fuel injectors when leaks are detected.
• Inspect the exhaust manifold, muffler, and exhaust pipe for leaks and other defects. Repairing minor leaks often involves replacing old seals, broken pipes and hoses, and fasteners. Sometimes repairs require professional welding.
• Diagnose the cause of engine misfires and repair the system.