p0420
Moderate
Fiat · Cars
Catalyst System Efficiency Below Threshold (Bank 1)
Quick Answer
Error code P0420 on your Fiat 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
Visually inspect catalytic converter for damage
2
Test oxygen sensors before and after catalytic converter
3
Perform fuel pressure test
4
Check for exhaust leaks with smoke test
5
Replace catalytic converter if defective
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 sensors
Gaskets
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