P0420 OBD2 Error Code: Catalyst System Efficiency Below Threshold (Bank 1)
What does this error code mean?
Catalyst System Efficiency Below Threshold (Bank 1)
Common symptoms
- Check engine light on
- Rotten egg smell from exhaust (sulphur)
- Slight increase in fuel consumption
- High HC/CO readings in emission test
What causes it?
- Worn or clogged catalytic converter
- Faulty or incorrect rear oxygen (O2) sensor
- Exhaust system leak (before or after oxygen sensor)
- Engine running consistently rich or lean (P0171, P0172, etc.)
How to fix
- Check exhaust leaks and oxygen sensors; replace rear O2 sensor if needed
- Fix mixture fault codes first if present
- Replace catalyst with OEM or quality aftermarket if confirmed inefficient
Is it safe to drive?
The vehicle can be driven; emissions may exceed limits and you might fail inspection. Performance is often not severely affected. If the catalyst clogs, exhaust backpressure increases and power drops. Get it diagnosed and repaired soon.
Estimated repair cost
Estimated cost range (labour and parts): $50 – $500 USD. Rear O2 sensor only: around 1500–3000 TL. Catalyst replacement: 5000–15000 TL depending on vehicle and parts.
Expert opinion
Check exhaust leaks and oxygen sensors first. Rear O2 sensors often fail or respond slowly; replacing them fixes many vehicles. If mixture codes (P0171, P0172) exist, fix those first—otherwise a new catalyst can be damaged. If the catalyst is truly worn out, replacement is required.
For information only. Content on this site is for general guidance; it does not replace professional diagnosis or repair. Always consult a qualified technician before making decisions about your vehicle.
Frequently asked questions
- Does P0420 always mean the catalyst?
- No. Rear O2 sensor, exhaust leaks, or mixture faults can also trigger this code. Rule those out before replacing the catalyst.
- Is it harmful to drive with P0420?
- Short term it does not seriously harm the engine; emissions rise and you may fail inspection. If the catalyst clogs, performance drops.
- How long does P0420 repair take?
- O2 sensor replacement: a few hours. Catalyst replacement: up to a day depending on parts availability.