P0420 on Honda: Catalyst System Efficiency Below Threshold (Bank 1)
Severity: 2 of 4
Usually safe to drive for a short time if the light is steady and the car runs normally. Fix the cause before it gets worse, because a converter that is overheating from misfires can become a fire risk and will fail completely. If the light flashes, or you smell strong sulfur or see a glowing red converter, park it.
Estimate range covers the repairs listed below; see the cost breakdown. A flashing check engine light, a hot temperature gauge, or brakes that feel different mean stop driving and get help.
What P0420 usually means on a Honda
Honda vehicles frequently set P0420 as they age. Many Honda engines have the primary converter mounted close to the exhaust manifold, which shortens heat up time but can make replacement more involved.
Misfires from fouled plugs on V6 models with cylinder deactivation can damage converters, so address misfire history first.
Most likely causes on Honda vehicles
- Exhaust leak
- Rear oxygen sensor
- Misfire history damaging the converter
- Converter wear
Order reflects cost and likelihood in general; your vehicle may differ. Confirm with testing before buying parts.
Honda specific tips
- Read misfire counters before buying a converter.
- Check your emissions warranty, which typically covers the converter for 8 years or 80,000 miles in the US.
Estimated repair cost
This is an estimate, built from typical parts price ranges plus labor hours times an assumed shop rate of $110 to $190 per hour. Actual prices vary by vehicle, region, and shop. Diagnosis fees are often extra.
| Repair | Parts | Labor | Estimate (low / high) |
|---|---|---|---|
| Rear oxygen sensor | $30 to $120 | 0.3 to 1.0 h | $30 + 0.3 × $110 = $63 $120 + 1.0 × $190 = $310 |
| Exhaust gasket or flex pipe repair | $15 to $150 | 0.5 to 1.5 h | $15 + 0.5 × $110 = $70 $150 + 1.5 × $190 = $435 |
| Catalytic converter (aftermarket, direct fit) | $250 to $1,400 | 1.0 to 2.5 h | $250 + 1.0 × $110 = $360 $1,400 + 2.5 × $190 = $1,875 |
Costs are generic for this code; parts and labor on some Honda models may run higher.
Watch a repair video
P0420 How To Diagnose A Bad Catalytic Converter (EricTheCarGuy). Third-party video, loads from YouTube (privacy-enhanced mode) only when you press play. Procedures vary by vehicle. Search YouTube for “P0420 Honda fix” videos.
Repair manuals
Step-by-step service manuals for Honda with torque specs and wiring diagrams:
Haynes manuals for Honda Chilton manuals
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Check Honda recalls
Enter your VIN at the NHTSA recall lookup before paying for repairs.
FAQ
Why does my Honda keep setting P0420 after a new converter?
Often the original cause, such as misfires or oil burning, was not fixed, or the replacement converter is low quality.
Should I go to the dealer?
If you are within the emissions warranty, yes.
Read the full P0420 guide or more Honda check engine notes.