P0420 on BMW
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 on BMW vehicles indicates catalyst system efficiency below threshold (bank 1). Au7o has documented this code across 1 BMW model — most commonly on X6. 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 BMW range from $4,900 to $12,000, 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 BMW by Model
BMW X6(1 issue)
- N63 Valve Stem Seal / Oil Consumption (xDrive50i)2008-2023
The N63 4.4L twin-turbo V8 engine in X6 xDrive50i across all generations (E71, F16, G06) suffers from premature valve stem seal degradation causing excessive oil consumption. The hot-vee configuration where turbos sit between the cylinder banks creates extreme heat that degrades the valve stem seals faster than normal. Oil consumption of 1 quart per 1,000 miles or more is common. BMW issued TSB MC-10149960-9999 acknowledging the issue. The repair is extremely labor-intensive requiring cylinder head removal. Elring seals 11340039494 ($80-120/set of 16, need 2 sets for V8) are OEM spec. 5150 AutoSport Viton seals ($150-200) are upgraded aftermarket option. AGA Tools kit AGA-N63-VSK-K ($500-800) includes specialized tools for the job.
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View P0420 across all makes →Frequently Asked Questions
What does P0420 mean on BMW?▼
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 BMW specifically, this code is documented across 1 model.
What causes P0420 on BMW vehicles?▼
Common causes on BMW: 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 BMW?▼
Repair costs on BMW range from $4,900 to $12,000, depending on the specific model and root cause.
Which BMW models have P0420 documented?▼
Au7o has documented P0420 on 1 BMW model: X6.