Pxxxx · Powertrain
P0420
Catalyst Efficiency Below Threshold (Bank 1)
ModeratePowertrain ECU
What it means
The downstream O2 sensor on Bank 1 is mirroring the upstream sensor too closely, indicating the catalytic converter is no longer effectively storing oxygen.
Common causes
Listed in rough order of frequency. Verify with vehicle-specific service info before parts replacement.
- 01Worn or damaged catalytic converter
- 02Failed downstream O2 sensor reporting inaccurate readings
- 03Engine running rich (carbon-fouling the cat)
- 04Exhaust leak between the upstream and downstream O2 sensors
- 05Aftermarket or low-grade replacement catalytic converter
Affected systems
ECUs and subsystems that report or are affected by this code.
Engine Control Module (ECM)Emissions
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