High Severity
P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)
P0420 is one of the most common and most dreaded OBD-II codes. It means the catalytic converter on Bank 1 is not converting exhaust pollutants efficiently enough. The downstream O2 sensor signal is too similar to the upstream sensor, indicating the catalyst is not doing its job.
Common Causes
- Worn or failed catalytic converter
- Faulty downstream O2 sensor giving false readings
- Engine misfire dumping unburned fuel into the cat
- Oil or coolant contamination in the exhaust
- Exhaust leaks before or after the converter
- Using leaded fuel or excessive oil consumption
Symptoms
- Check engine light on
- Failed emissions test
- Reduced engine performance (in severe cases)
- Sulfur or rotten egg smell from exhaust
- Usually no immediate driveability issues
How to Fix P0420
- Diagnose before replacing the cat. Check O2 sensor data first
- Replace downstream O2 sensor if signal is erratic
- Fix any misfires or oil burning first
- Replace catalytic converter if confirmed failed
- Check for exhaust leaks and repair
- Some vehicles respond to a catalytic converter cleaner additive
What Does P0420 Mean?
The P0420 diagnostic trouble code (DTC) is an OBD-II code stored by your vehicle's Engine Control Unit (ECU) when it detects catalyst system efficiency below threshold (bank 1). This code is part of the Catalytic Converter system. When your check engine light comes on and a scan tool reads P0420, it means your vehicle needs attention in this area.
OBD-II codes starting with P0 are generic powertrain codes defined by the SAE standard, meaning P0420 has the same basic definition regardless of vehicle make or model. However, the specific causes and repair procedures can vary between manufacturers.
Can I Drive With Code P0420?
With a P0420 code, you should address this issue as soon as possible. Continued driving may cause damage to your catalytic converter, engine, or other components. If your check engine light is flashing, pull over safely and have the vehicle towed. A flashing light means active misfires that can cause immediate catalytic converter damage.