p0420
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Audi · SUVs
Catalyst System Efficiency Below Threshold (Bank 1)
Quick Answer
Error code P0420 on your Audi SUVs means: Catalyst System Efficiency Below Threshold (Bank 1). This is usually fixable — see the step-by-step guide below.
Affected Models
Likely Causes
How to Fix It
1
Check for any other pending codes that could cause secondary catalyst failure
2
Compare upstream and downstream O2 sensor waveforms to verify catalyst efficiency
3
Inspect the exhaust system for leaks at manifold gaskets and flex pipes
4
Confirm no engine misfires or oil burning conditions exist
5
Replace the catalytic converter if diagnostic testing confirms substrate failure
Pro Tip
This issue typically requires professional diagnosis. Attempting this fix without proper training could cause further problems.
Parts You May Need
Catalytic converter (bank 1)
Downstream O2 sensor
Exhaust gaskets
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