OBD-II code · Powertrain (generic)
P0420 Catalyst System Efficiency Below Threshold (Bank 1)
The oxygen storage capacity of the main catalytic converter on bank 1 has degraded below the OBD threshold. The downstream oxygen sensor signal increasingly mirrors the upstream sensor, indicating reduced conversion efficiency.
Is it safe to drive with P0420?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
Common causes
- Most likelyCatalyst aged or contaminated
- PossibleDownstream oxygen sensor drift
- Less likelyExhaust leak before catalyst
- Less likelyLong-term misfire damage
Symptoms
- Check engine light on
- Failed emissions inspection
- Possibly slightly increased fuel consumption
- Sulfur (rotten egg) smell from exhaust under load
Repair at a glance
- Difficulty
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 1.5–5 h
- Emissions test
- Likely to fail
- After the fix
- A drive cycle is needed before the light stays off
- Parts involved
- Catalytic Converter, Oxygen Sensor Upstream, Oxygen Sensor Downstream
Related codes
- P0430Catalyst System Efficiency Below Threshold (Bank 2)
- P041DEGR Temperature Sensor B Circuit High
- P041EEGR Temperature Sensor B Circuit Intermittent/Erratic
- P041FSecondary Air Injection System Switching Valve A Circuit Low
- P0421Warm Up Catalyst Efficiency Below Threshold (Bank 1)
- P0422Main Catalyst Efficiency Below Threshold (Bank 1)
- P0423Heated Catalyst Efficiency Below Threshold (Bank 1)
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.