The Quick Answer above is the generic definition. What follows is what is documented for the BMW models that log P0138, checked against manufacturer data and owner reports.
DTC/P0138·BMW
P0138 on BMW: Causes, Symptoms and Fix Cost
Quick Answer
Moderate SeveritySafe to drive (short-term)BMW
P0138 on a BMW means the downstream oxygen sensor on Bank 1 (post-catalyst, Sensor 2) is reporting a voltage that is too high for too long, above roughly 1.0V. A healthy downstream sensor reads around 0.6–0.8V steady. Persistent high voltage typically indicates a rich exhaust condition, a short to voltage in the wiring, or a contaminated sensor.
Repair cost on BMW€65 - €650
Connect any standard Bluetooth OBD2 adapter to your BMW and Skanyx tells you what P0138 means on that engine, whether it is safe to keep driving, and what the fix should cost. Free on iPhone and Android.
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BMW 3 Series
2006–2024On the N20 and N55, leaking fuel injectors are a known cause of real rich conditions that trigger P0138. The N20 has a documented issue with the high-pressure pump and injector seals around 100,000 km. BMW OEM Bosch sensors run 110–200 euros, aftermarket Bosch (identical) around 60–110 euros. The downstream sensor is accessible from under the car.
What causes P0138 on a BMW?
Beyond the generic causes listed on the main P0138 page, these are the documented BMW-specific patterns:
BMW 3 SeriesN20 leaking fuel injectors causing genuine rich condition
BMW 3 SeriesN55 HPFP wear at 120,000+ km elevating fuel pressure
How to diagnose P0138 on a BMW with OBD2
Follow these steps to pinpoint the root cause of P0138:
- Read freeze frame data and check for companion codes (P0172, P0175, P0420) that point to a real rich condition or converter problem
- Use a scan tool to view downstream sensor voltage. A reading stuck above 0.9V at idle indicates a rich condition or a faulty sensor
- Check long-term fuel trims. If LTFT is significantly negative (under -10%), the engine is genuinely running rich
- Inspect the Bank 1 Sensor 2 signal wire for shorts to voltage, especially near heat shields and the battery positive cable
- Substitute a known-good sensor. If high voltage persists, the issue is fuel mixture or wiring, not the sensor
How much does P0138 cost to fix on a BMW? (EUR)
Estimated repair costs for P0138 in EUR, compiled across makes. Expect BMW parts and labour towards the upper end of each range.
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
Related codes that often appear with P0138 on BMW
These codes commonly cluster with P0138 on BMW vehicles:
Looking for the full P0138 reference (all makes, full diagnosis flow, complete repair cost matrix)?
See the main P0138 guideSkanyx · BMW
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