P0420

What this code means

The downstream oxygen sensor shows the catalytic converter on bank 1 is no longer cleaning exhaust gases efficiently.

Common symptoms

• Check engine light, often no other symptoms
• Slightly reduced fuel economy
• Sulfur smell from exhaust in some cases

Most common causes

• Aged or failing catalytic converter
• Faulty upstream or downstream O2 sensor
• Exhaust leak before the sensor
• Engine misfires or oil burning damaging the catalyst