P0420

What it means

Your car's computer thinks the catalytic converter on the engine's first bank isn't cleaning exhaust gases as well as it should, which means it's not working efficiently.

Is it safe to drive?

While often not an immediate breakdown risk, prolonged driving with a P0420 can lead to further catalytic converter damage, increased emissions, and potentially reduced engine performance and fuel economy. It's best to diagnose and repair promptly.

Most likely causes

• Failing Catalytic Converter

  high

  This is the most common cause, especially on older vehicles or those with high mileage, indicating the catalyst material is degraded.

• Oxygen Sensor (O2 Sensor) Malfunction

  medium

  A faulty upstream or downstream O2 sensor can send incorrect readings, leading the PCM to falsely believe the converter is inefficient.

• Exhaust Leak

  medium

  Leaks in the exhaust system before or between the O2 sensors can allow unmetered air in, affecting sensor readings and catalyst efficiency calculations.

• Engine Misfires or Fuel System Issues

  low

  Consistent misfires or rich/lean fuel conditions can damage the catalytic converter over time by sending unburnt fuel or excessive heat into it.

Related help

• Full P0420 diagnosis, causes & costs
• How to read check engine light codes
• Is it safe to drive with the light on?