P0420 on Honda Civic FC: Catalyst System Efficiency Below Threshold (Bank 1)
Short answer: P0420 means “Catalyst System Efficiency Below Threshold (Bank 1)”. The car’s computer has found a fault in the Emissions system and turns on the Check engine light (MIL). First check: read the code and live data with an OBD-II scanner.
Standard OBD-II code Same meaning on all cars (SAE J2012). Not checked against this car’s factory manual.
- Code
- P0420
- System
- Emissions
- Dashboard light
- Check engine light (MIL)
- Applies to
- Generic OBD-II meaning. Not every code is used by this car.
- Confidence
- medium (generic OBD-II definition, not checked against the factory manual for this car)
- Source
- obd-codes.com
Symptoms and when it is set
- Check engine light on
- Symptoms. For example, if the substance inside the catalytic converter is broken or failed, it may be restricting the exhaust which will result in…
Likely causes
- A code
- P0420
- Has happened
- Leaded fuel was used where unleaded was called for (unlikely)
- A damaged or failed oxygen / O2 sensor
- Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
- The engine coolant temperature sensor is not working properly
What to check, in order
- Read the code and live data with an OBD-II scanner
- Look for exhaust or vacuum leaks and check the wiring of the sensor or valve
- Test the part before replacing it
- Fix the cause, clear the code and test-drive
Related on this car
Same code on other cars
- P0420 · Toyota Passo (1st gen)
- P0420 · Toyota Vitz (1st gen)
- P0420 · Toyota Corolla (11th gen, E170)
- P0420 · Toyota Corolla (10th gen, E140)
- P0420 · Honda City (5th gen, GM2)
- P0420 · Suzuki Wagon R (Pakistan, K10B)
- P0420 on all cars compared
Clearing the code does not fix the fault. If it comes back, the problem is still there. For brakes, airbags and steering, see a trained mechanic.