P0420: catalyst efficiency below threshold, bank 1

What the code actually means

The control unit judges the converter by comparing the oxygen sensors before and after it. P0420 means the result is below the expected level on bank 1, the side of the engine where cylinder 1 sits. P0430 is the same code for bank 2 on a V engine. Most four cylinder engines have only one bank.

Common causes

• A catalytic converter on its way out
• A misfire upstream, making the converter work harder
• A failing oxygen sensor, giving an incorrect reading

What to do now

1. Have the car scanned for other codes stored alongside it, especially misfire codes.
2. If the check engine light is flashing, slow down. With a misfire, that means unburned fuel is reaching the converter.
3. Ask for the oxygen sensors to be assessed before the converter is condemned.
4. Do not clear the code before the cause is found. Clearing erases the freeze frame data that helps find it.

What people often get wrong

Assuming P0420 means the converter has failed. A code points to a system, not always the exact cause. Finding the root cause before replacing the converter, one of the pricier emissions parts, can save a significant repair bill.

When the car must go to a workshop

Diagnosing catalytic converter issues is worth leaving to a qualified technician with the right tools. Guessing and replacing parts one at a time is often more expensive than a proper diagnosis up front.

Read next

• P0300: misfire on more than one cylinder
• P0135: the oxygen sensor heater circuit
• P0171: system too lean

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.