Complete List of P0420 Code Causes

The Most Common Causes

• Failing Catalytic Converter: This is the most likely culprit, especially on vehicles with over 100,000 miles. The precious metals inside degrade over time and can no longer effectively convert harmful gases.
• Faulty Rear Oxygen (O2) Sensor: The downstream O2 sensor's job is to monitor the converter. If the sensor is bad, it can send incorrect data to the car's computer, falsely triggering the P0420 code.
• Exhaust System Leaks: Any leak between the engine and the catalytic converter (like a cracked exhaust manifold or a bad gasket) can let oxygen into the exhaust stream. This extra oxygen will confuse the O2 sensors and can trigger the code.
• Engine Misfires: If your engine is misfiring, it's sending unburnt fuel into the exhaust. This fuel can ignite inside the catalytic converter, causing it to overheat and melt internally, leading to permanent damage.
• Rich or Lean Fuel Condition: Issues like leaking fuel injectors or a faulty mass airflow sensor can throw off the engine's air-fuel mixture. A prolonged rich or lean condition can contaminate and clog the catalytic converter.
• Contaminated Fuel or Oil Burning: Using leaded fuel (in older cars), excessive fuel additives, or having an engine that burns oil can lead to deposits that coat the inside of the converter, rendering it useless.