p0420
Moderate
Alfa Romeo · Sedans & Sports Cars
Catalyst System Efficiency Below Threshold (Bank 1)
Quick Answer
Error code P0420 on your Alfa Romeo Sedans & Sports Cars 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 oxygen sensor readings are within range
2
Check exhaust system for leaks
3
Measure catalytic converter efficiency
4
Clear code and perform highway test drive
Pro Tip
This fix requires some technical knowledge. If you're unsure about any step, consider contacting support or a qualified technician.
Parts You May Need
Catalytic converter
Oxygen sensor
Exhaust gasket
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