p0420
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Volkswagen · Sedans
Catalyst System Efficiency Below Threshold Bank 1
Quick Answer
Error code P0420 on your Volkswagen 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
Likely Causes
How to Fix It
1
Verify no other DTCs are present that could cause secondary catalyst damage (misfires, rich/lean codes)
2
Compare upstream and downstream O2 sensor waveforms; downstream should be relatively flat if the cat is healthy
3
Inspect exhaust system for leaks at the manifold, downpipe, and catalytic converter flanges
4
Check catalytic converter substrate temperature differential (inlet vs. outlet) to assess converter efficiency
5
If the converter is confirmed failed, replace with an OEM or CARB-compliant catalytic converter
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 O2 sensor
Exhaust gaskets
Exhaust clamps
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