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p0420
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Toyota · Sedans
Catalyst System Efficiency Below Threshold Bank 1
Quick Answer
Error code P0420 on your Toyota Sedans means: Catalyst System Efficiency Below Threshold Bank 1. This is usually fixable — see the step-by-step guide below.
Affected Models
Applies to all models
Avalon XLE 2022Camry XSE 2024Camry Hybrid SE 2024Corolla SE 2024Corolla Hybrid LE 2024Crown Platinum 2024Mirai Limited 2024Prius XLE 2024Camry LE 2023Camry LE 2024Camry SE 2023Camry SE 2024Camry TRD 2024Camry XLE 2024Corolla LE 2023Corolla LE 2024Corolla XSE 2024Crown Limited 2024Crown XLE 2024Prius LE 2024Prius Limited 2024
Likely Causes
  1. Worn or contaminated catalytic converter
  2. Faulty downstream oxygen sensor (Bank 1 Sensor 2)
  3. Exhaust leak before or after the catalytic converter
  4. Engine misfires causing unburned fuel to damage the catalyst
  5. Use of leaded fuel or excessive oil consumption contaminating the catalyst
How to Fix It
1
Verify no other DTCs are present that could cause secondary catalyst damage
2
Inspect exhaust system for leaks from manifold to tailpipe
3
Compare upstream and downstream O2 sensor waveforms with a scan tool
4
Check catalytic converter inlet and outlet temperatures to confirm reduced efficiency
5
Replace catalytic converter if confirmed failed and address any root cause misfires
Pro Tip
This issue typically requires professional diagnosis. Attempting this fix without proper training could cause further problems.
Parts You May Need
Catalytic converter
Downstream oxygen sensor
Exhaust gaskets
Exhaust clamps
Related Error Codes
P0171P0300P0301P0441P0442
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