P0420 on Chevrolet: 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 Chevrolet
Chevrolet cars and trucks often set P0420 at high mileage. On V8 trucks with cylinder deactivation, misfires from lifter problems can damage converters, so look at misfire history.
Exhaust manifold leaks on high mileage trucks can also affect the reading.
Most likely causes on Chevrolet vehicles
- Exhaust manifold leak
- Rear oxygen sensor
- Misfire or oil consumption
- Worn converter
Order reflects cost and likelihood in general; your vehicle may differ. Confirm with testing before buying parts.
Chevrolet specific tips
- Check for misfire codes and lifter noise on V8 trucks.
- Check the emissions warranty if eligible.
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 Chevrolet 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 Chevrolet fix” videos.
Repair manuals
Step-by-step service manuals for Chevrolet with torque specs and wiring diagrams:
Haynes manuals for Chevrolet Chilton manuals
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Check Chevrolet recalls
Enter your VIN at the NHTSA recall lookup before paying for repairs.
FAQ
Do Chevy trucks have two converters?
Many V8 trucks have one per bank, so you may see P0420, P0430, or both.
Read the full P0420 guide or more Chevrolet check engine notes.