p0420
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Honda · Sedans
Catalyst System Efficiency Below Threshold Bank 1
Quick Answer
Error code P0420 on your Honda 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
CV3CV4FC1FC4FE1FL5ZC5Accord EX 2024Accord EX-L 2024Accord Hybrid EX-L 2024Accord Hybrid Sport 2024Accord Hybrid Touring 2024Accord LX 2024Accord Sport 2024Accord Touring 2024Civic EX 2024Civic Hybrid Sport 2025Civic Hybrid Sport Touring 2025Civic LX 2024Civic Si 2024Civic Sport 2024Civic Touring 2024Civic Type R 2024ZE4
Likely Causes
How to Fix It
1
Verify no other DTCs are present that could cause secondary catalyst damage (misfires, rich/lean codes)
2
Inspect the downstream O2 sensor waveform with a scan tool; it should show minimal switching compared to upstream
3
Check for exhaust leaks at the manifold, flex pipe, and catalytic converter connections
4
Measure catalyst inlet and outlet temperatures; outlet should be hotter than inlet during normal operation
5
If the catalytic converter is confirmed failed, replace with a CARB-compliant or OEM unit
Pro Tip
This issue typically requires professional diagnosis. Attempting this fix without proper training could cause further problems.
Parts You May Need
Catalytic converter (OEM or CARB-compliant)
Downstream oxygen sensor
Exhaust gaskets and hardware
Exhaust clamps
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