P0420: Catalyst System Efficiency Below Threshold (Bank 1)
Applies to: Universal OBD-II (Subaru, Toyota, Honda, Ford, Nissan)
Can you still drive / use device? Yes for moderate distances, but vehicle will fail state emissions inspections.
Common Symptoms
- Illuminated Check Engine Light (steady)
- Rotten egg / sulfur odor from exhaust
- Slight decrease in fuel efficiency and sluggish highway passing power
- Vehicle fails state OBD-II emissions smog test
Root Causes
- Catalytic converter internal substrate degraded or melted
- Faulty Downstream Oxygen Sensor (O2 Sensor Bank 1 Sensor 2)
- Exhaust leak upstream of the catalytic converter
- Engine burning oil or coolant contaminating the catalyst
Step-by-Step Instructions to Fix P0420
Check Live O2 Sensor Voltage Data
Use a live-data scan tool. Downstream O2 sensor should show a steady voltage around 0.45V to 0.7V. If it fluctuates rapidly between 0.1V and 0.9V like the upstream sensor, the cat is not storing oxygen.
Inspect Exhaust Pipes for Upstream Leaks
Check exhaust manifold gaskets and flex pipes before the catalytic converter. Extra air entering exhaust false-triggers the P0420 code.
Test and Replace Downstream O2 Sensor
If the sensor is sluggish or older than 100,000 miles, replace the downstream oxygen sensor first before condemning the expensive catalytic converter.
Try Catalytic Converter Cleaner Solution
Pour a designated catalytic cleaner (e.g. Cataclean) into 1/4 tank of fuel and drive 50 miles on the highway to burn off surface carbon deposits.
Replace Catalytic Converter Assembly
If codes return after sensor replacement and cleaning, replace the catalytic converter with an EPA or CARB compliant unit.
How to Prevent P0420 in the Future
- Fix engine misfires immediately to prevent raw fuel from burning out the catalytic converter.
- Fix oil-burning leaks promptly (bad PCV valve or valve stem seals).