P0138 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.
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€91 - €910
What does P0138 mean on a BMW?
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 BMW-specific patterns we see most often:
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 on a BMW (BMW parts and labour typically run 40% above the average for this code).
Prices estimated as of May 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:
FAQ: P0138 on BMW
Can I drive with P0138?
Short term yes, but a real rich condition wastes fuel and can clog the catalytic converter within weeks of constant rich running. If you smell strong fuel from the exhaust or fuel economy has dropped sharply, repair it promptly.
What causes high voltage on a downstream O2 sensor?
A genuinely rich exhaust (leaking injector, high fuel pressure, stuck-open fuel regulator), a shorted sensor signal wire, or a contaminated sensor element. Always check fuel trims and look for companion codes before replacing the sensor.
Can a bad O2 sensor cause rough idle?
The downstream sensor alone usually does not cause a rough idle because it does not drive primary fuel mixture decisions. If you have rough idle plus P0138, look for an upstream sensor issue, a vacuum leak, or a misfire that is also triggering the rich downstream reading.
How do I know if my catalytic converter is bad versus the sensor?
Compare upstream and downstream waveforms. If the downstream sensor switches as rapidly as the upstream, the converter has lost its oxygen storage and is failing. If the downstream is just stuck high regardless of operating conditions, the sensor is the likely culprit.
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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