P0420 on MINI
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 on MINI vehicles indicates catalyst system efficiency below threshold (bank 1). Au7o has documented this code across 1 MINI model — most commonly on GP. P0420 means the engine computer determined the catalytic converter on Bank 1 isn't cleaning the exhaust as efficiently as it should. The computer compares the upstream and downstream oxygen sensors; a healthy converter stores and releases oxygen, so the downstream sensor should read fairly steady, while a worn one lets its readings mirror the upstream sensor. When that efficiency falls below a set threshold, this code is set. While it most often points to a worn-out catalytic converter, it can also be triggered by upstream problems like a faulty oxygen sensor or an exhaust leak, so the converter isn't always the actual fault. Typical repair costs on MINI range from $800 to $3,000, depending on the specific model and root cause.
Common Causes of P0420
- •Worn-out or failing catalytic converter (Bank 1)
- •Faulty or aged downstream (or upstream) oxygen sensor
- •Exhaust leak before or near the oxygen sensors
- •Engine running rich or lean (fuel trim issues)
- •Misfires or oil/coolant contaminating the converter
- •Damaged or aftermarket low-quality catalytic converter
- •Faulty oxygen sensor wiring or connectors
- •Software/calibration needing update (on some vehicles)
P0420 on MINI by Model
MINI GP(1 issue)
- Cracked Exhaust Manifold (Turbo Models)2013-2023
The cast iron exhaust manifold (integrated with the turbo housing on the N18/B48) is prone to cracking from thermal stress. The GP engines produce higher exhaust gas temperatures than standard MINI models due to their aggressive tuning. Cracks develop at the cylinder-to-manifold flanges, causing exhaust leaks, a ticking noise, and potential turbo bearing damage from escaping exhaust gases. Track use and hard driving significantly accelerate cracking.
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View P0420 across all makes →Frequently Asked Questions
What does P0420 mean on MINI?▼
P0420 stands for "Catalyst System Efficiency Below Threshold (Bank 1)." P0420 means the engine computer determined the catalytic converter on Bank 1 isn't cleaning the exhaust as efficiently as it should. The computer compares the upstream and downstream oxygen sensors; a healthy converter stores and releases oxygen, so the downstream sensor should read fairly steady, while a worn one lets its readings mirror the upstream sensor. When that efficiency falls below a set threshold, this code is set. While it most often points to a worn-out catalytic converter, it can also be triggered by upstream problems like a faulty oxygen sensor or an exhaust leak, so the converter isn't always the actual fault. On MINI specifically, this code is documented across 1 model.
What causes P0420 on MINI vehicles?▼
Common causes on MINI: Worn-out or failing catalytic converter (Bank 1), Faulty or aged downstream (or upstream) oxygen sensor, Exhaust leak before or near the oxygen sensors, Engine running rich or lean (fuel trim issues), Misfires or oil/coolant contaminating the converter. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0420 on a MINI?▼
Repair costs on MINI range from $800 to $3,000, depending on the specific model and root cause.
Which MINI models have P0420 documented?▼
Au7o has documented P0420 on 1 MINI model: GP.