p0420
Moderate
MINI · Hatchbacks & Crossovers
Catalyst System Efficiency Below Threshold
Quick Answer
Error code P0420 on your MINI Hatchbacks & Crossovers means: Catalyst System Efficiency Below Threshold. This is usually fixable — see the step-by-step guide below.
Affected Models
Likely Causes
How to Fix It
1
Measure catalytic converter temperature before and after
2
Perform oxygen sensor diagnostic tests
3
Check fuel trims and oxygen sensor switching rates
4
Inspect exhaust system for leaks
5
Evaluate engine load and fuel injection parameters
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
Exhaust gaskets
Thermal imaging camera
OBD-II scanner with fuel trim monitoring
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