P0420 on Subaru: 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 Subaru
P0420 is a common Subaru code. Subaru exhausts place the front converter close to the engine and the rear sensor right after it, so small exhaust leaks in the crossover area can affect the test.
Most likely causes on Subaru vehicles
- Exhaust leak at the crossover or flex joint
- Rear oxygen sensor
- Misfires from head gasket issues on older EJ25 engines
- Worn converter
Order reflects cost and likelihood in general; your vehicle may differ. Confirm with testing before buying parts.
Subaru specific tips
- Rattling heat shields are common on Subarus but are separate from the code.
- Check coolant level for signs of head gasket leakage on older models.
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 Subaru 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 Subaru fix” videos.
Repair manuals
Step-by-step service manuals for Subaru with torque specs and wiring diagrams:
Haynes manuals for Subaru Chilton manuals
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Check Subaru recalls
Enter your VIN at the NHTSA recall lookup before paying for repairs.
FAQ
Can a Subaru head gasket cause P0420?
A head gasket leak that burns coolant can contaminate the converter.
Read the full P0420 guide or more Subaru check engine notes.