OBD-II Code: P0420

Catalyst System Efficiency Below Threshold (Bank 1)

P0420 means the catalytic converter on Bank 1 is not operating efficiently. The oxygen sensors before and after the catalytic converter show similar readings, indicating the converter is not reducing exhaust pollutants as expected.

Can I Drive?

Usually short-distance only; diagnose soon
Stop driving if you notice overheating, bad misfires, loss of power, fuel smell, or flashing warning lights.

Common Causes

• Failing or damaged catalytic converter
• Exhaust leaks before the catalytic converter
• Faulty upstream (pre-cat) oxygen sensor
• Faulty downstream (post-cat) oxygen sensor
• Rich fuel mixture from other issues

DIY Checks

• Inspect exhaust system for leaks before the catalytic converter
• Check oxygen sensor wiring and connectors for damage
• Use an OBD-II scanner to monitor live O2 sensor voltage data (should switch rapidly before converter, slowly after)

Mechanic Questions to Ask

Was the catalytic converter efficiency tested? Can you show me the live O2 sensor data before and after the converter? Is the issue the converter itself or an upstream sensor causing false readings?

Related Codes

p0430 — Bank 2 | p0141 — O2 Sensor Heater