P0420
Catalyst System Efficiency Below Threshold (Bank 1)
Moderatecatalyst
Overview
The P0420 code signals that the PCM has detected a catalyst system efficiency below threshold (bank 1) condition. This code relates to the catalytic converter, a critical emissions component that converts harmful exhaust gases into less harmful substances. Reduced catalyst efficiency typically means the converter is no longer adequately processing exhaust pollutants.
Symptoms
- Rattling noise from under the vehicle
- Reduced engine performance
- Decreased fuel economy
- Engine running hotter than normal
Common Causes
- Contaminated catalytic converter (leaded fuel, silicone)
- Faulty oxygen sensor providing incorrect readings
- Rich or lean fuel mixture over extended period
- Oil or coolant entering the exhaust system
Diagnosis Steps
- 1Connect an OBD-II scanner and read all stored trouble codes and freeze frame data
- 2Monitor upstream and downstream O2 sensor readings; compare switching patterns
- 3Check for exhaust leaks before the catalytic converter
- 4Inspect for engine misfires or other codes that could damage the converter
- 5Test the catalytic converter efficiency using a temperature gun (inlet vs outlet)
Estimated Repair Cost
$200 — $2,500
Cost varies by vehicle, location, and repair shop. Includes parts and labor.
Commonly Affected Vehicles
Toyota CamryHonda CR-VHyundai TucsonHonda Accord
Related Codes
More Catalyst Codes
P0421Warm Up Catalyst Efficiency Below Threshold (Bank 1)
ModerateP0422Main Catalyst Efficiency Below Threshold (Bank 1)
ModerateP0423Heated Catalyst Efficiency Below Threshold (Bank 1)
ModerateP0424Heated Catalyst Temperature Below Threshold (Bank 1)
ModerateP0425Catalyst Temperature Sensor (Bank 1 Sensor 1)
LowP0426Catalyst Temperature Sensor Range/Performance (Bank 1 Sensor 1)
LowFrequently Asked Questions
Catalytic converter replacement typically costs $500-$2,500 depending on the vehicle and converter type. OEM converters are more expensive but often provide better performance and longevity.