p0420
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SEAT · SUVs & Crossovers
Catalyst System Efficiency Below Threshold (Bank 1)
Quick Answer
Error code P0420 on your SEAT SUVs & Crossovers 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
Verify no other codes are present that could cause secondary catalyst damage
2
Compare upstream and downstream O2 sensor waveforms on bank 1
3
Inspect the exhaust system for leaks near the catalytic converter
4
Check for engine misfires or oil burning that may have damaged the catalyst
5
Replace the catalytic converter if efficiency testing confirms 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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