Fault code P0421 – a step-by-step guide to diagnosing and fixing it

Error code P0421 is displayed and the check engine light is on - what does this mean?

DTC P0421 is triggered by the OBD/EOBD system when the efficiency of the catalytic converter is below the expected threshold. This trouble code indicates that the catalytic converter in bank 1 is not functioning properly and exhaust gas purification is impaired. For some manufacturers, this fault code can also occur as a manufacturer-specific variant.

What can cause this fault code?

• Defective or worn catalytic converter
• Faulty lambda sensors upstream or downstream of the catalytic converter
• Damaged cable connections to the oxygen sensors
• Leaks in the exhaust system
• Engine problems such as misfiring or incomplete combustion
• Contamination of the catalytic converter due to poor fuel quality
• Defective fuel injection or ignition system
• Loose or corroded plug connections

What symptoms occur in parallel with fault code P0421?

• Loss of power: The vehicle shows reduced engine power and poor acceleration
• Increased fuel consumption: The engine requires more fuel than usual
• Unsteady idling: The engine runs unevenly or rough at idle speed
• Exhaust odor: Unusual smells from the exhaust, especially of rotten eggs
• Starting problems: The vehicle is harder to start than usual
• Jerking: Jerky ride especially at low engine speeds

What symptoms occur in parallel with fault code P0421?

• Systematic diagnostic steps should be carried out to rectify fault code P0421.
• Catalytic converter replacement: Replace the defective catalytic converter with a new or regenerated part, as this is the most common cause of this code
• Check and replace lambda sensors: Checking all oxygen sensors with a multimeter and replacing defective sensors, as faulty readings affect catalytic converter diagnostics
• Exhaust system inspection: Completely check the exhaust system for leaks, cracks or loose connections and repair accordingly
• Carry out engine diagnostics: Fix basic problems such as misfiring, faulty spark plugs or fuel injection issues that can damage the catalytic converter
• Repair wiring: Checking and repairing all cable connections and connectors for the lambda sensors and the EOBD system
• Clean fuel system: Professional cleaning of the fuel system and use of catalytic converter cleaners as a first measure for light soiling