Engine light came on. We get two codes, both showing P0420. Since this code came up twice, does this mean that the catalytic converter needs to be replaced? Or should I try replacing the downstream oxygen sensor first?
The most common reason for code P0420 is a failed catalytic converter. The code appears because the sensor has detected this failure. Now of course you would want to rule out that the sensor is functioning. This can be done with a scan tool by looking at the voltage readings. You might want to rule out the less expensive options before replacing any parts.
Saturn Code P0420
Description: Catalyst System Efficiency Below Threshold Bank 1
P0420 Probable Causes:
- Sensor or solenoid faulty
- Connector terminal contact is damaged or corroded
- Wire harness – Check harness for correct voltage, open, short to ground or short to voltage
- Update control unit software – Check for the latest control unit update
- Powertrain or Engine Control Module faulty
- HO2S sensor contamination/damaged
- Exhaust leaks or restrictions
- Catalytic converter damage
- Catalytic converter improper operation