Catalyst System Efficiency Below Threshold (Bank 1)
Emission · Catalytic Converter
Your catalytic converter is not cleaning exhaust gases efficiently enough. This code is common in vehicles over 80k miles and often appears after an upstream oxygen sensor failure or prolonged use of low-quality fuel.
- Worn or failing catalytic converterMost common
- Faulty O2 sensor (upstream or downstream)Common
- Exhaust leak near O2 sensor bungLess common
- Engine misfires contaminating converterLess common
Vehicles most commonly affected by P0420
Most commonly reported on Toyota Camry (2007–2011), Honda Accord (2003–2007), Subaru Outback (2005–2009), and Ford Ranger (2000–2005). Vehicles in the 100,000–150,000 mile range are most frequently affected as the catalytic converter approaches the end of its designed lifespan.
Real-world P0420 diagnosis scenario
The most common real-world P0420 scenario: a 2009 Toyota Camry with 118,000 miles throws the code on the highway. The owner replaces the upstream O2 sensor for $180 — the code returns in two weeks. The actual cause was the converter itself, degraded from years of short-trip city driving that never fully heated the catalyst. The lesson: always check both O2 sensor voltages with a live scan tool before replacing anything — if the downstream sensor is genuinely flat-lining rather than switching, the converter is the problem.
Before replacing the catalytic converter, perform a fuel trim check. If long-term fuel trims are above +10% on Bank 1, there is a lean condition that caused the converter to fail — and will destroy the new one too. Fix the lean condition first.
Sponsored: MyOBDCode may earn compensation when you click partner links on this page.
Frequently asked questions about P0420
What does P0420 mean?
Your catalytic converter is not cleaning exhaust gases efficiently enough. This code is common in vehicles over 80k miles and often appears after an upstream oxygen sensor failure or prolonged use of low-quality fuel.
Is P0420 serious?
Safe to drive short-term — fix within 2 weeks to avoid emissions failure.
How much does it cost to fix P0420?
Repair costs for P0420 range from $120 – $300 depending on the root cause and your vehicle.
Can I drive with P0420?
You can drive short distances but should have it repaired within 1–2 weeks to prevent further damage.
Can I pass an emissions test with P0420?
No. P0420 causes the catalyst efficiency monitor to fail, which is an automatic emissions test failure in all OBD-II states. You must repair the underlying fault and allow the catalyst monitor to complete before testing.
How long can I drive with P0420?
The car is driveable — P0420 does not cause mechanical damage in the short term. However, the longer you wait, the more the degraded converter affects fuel economy. Fix within 2–4 weeks.
Will a new gas cap fix P0420?
No. A gas cap fixes EVAP codes (P0440, P0442, P0455). P0420 is specifically about catalytic converter efficiency and is unrelated to the fuel cap.