P0420: Catalyst System Efficiency Below Threshold (Bank 1)
P0420 is the classic 'your cat is dying' code, but in 40%+ of cases the fix is actually a $250 oxygen sensor, not a $2,000 converter. Always rule out cheaper causes first.
Severity: moderate · System: Powertrain · Safe to drive: usually, short-term. Not urgent, but shops often push the $2K+ OEM cat. Ask for aftermarket EPA-compliant options first.
Symptoms
- Check engine light
- Reduced fuel economy
- Rotten-egg smell (rare)
Common causes
- Failing catalytic converter
- Bad downstream O2 sensor
- Exhaust leak
- Engine misfire damaging cat
Fair repair prices
| Downstream O2 sensor | $180–$400 |
|---|---|
| Exhaust leak repair | $100–$500 |
| Catalytic converter (aftermarket) | $500–$1,200 |
| Catalytic converter (OEM) | $1,200–$2,500 |
Ranges are US market averages (parts and labor). If your shop's quote is above the high end, ask for a line-item breakdown or run it through the invoice checker.
Other codes
- P0300 — Random / Multiple Cylinder Misfire Detected
- P0301 — Cylinder 1 Misfire Detected
- P0430 — Catalyst System Efficiency Below Threshold (Bank 2)
- P0171 — System Too Lean (Bank 1)
- P0174 — System Too Lean (Bank 2)
- P0442 — Evaporative Emission Control System Leak (Small)
- P0455 — Evaporative Emission Control System Leak (Large)
- P0128 — Coolant Temperature Below Thermostat Regulating Temperature