P0138 on Mercedes-Benz: Causes, Symptoms and Fix Cost
Quick Answer
Safe to drive (short-term)
P0138 on a Mercedes-Benz 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.
- Severity
- Moderate · Emissions
- Typical repair cost, all makes
- €65-€650
Connect a standard OBD2 adapter (Bluetooth LE on iPhone) to your Mercedes-Benz 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.
Get Skanyx freeWhat does P0138 mean on a Mercedes-Benz?
The Quick Answer above is the generic definition. What follows is the model-specific detail for the Mercedes-Benz models that log P0138.
Mercedes C-Class
2007-2024
On the W204 C-Class with the M271 1.8 Kompressor petrol, P0138 is either a genuine rich condition (check fuel trims and injectors) or the downstream sensor or its wiring ageing on a high-mileage car. The sensor runs roughly 70 to 220 euros depending on where you buy it.
What causes P0138 on a Mercedes-Benz?
Beyond the generic causes listed on the main P0138 page, these are the patterns noted for Mercedes-Benz models:
Genuine rich condition from a leaking injector
: Mercedes C-Class
Downstream sensor ageing on high-mileage cars
: Mercedes C-Class
How to diagnose P0138 on a Mercedes-Benz with OBD2
- 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 Mercedes-Benz? (EUR)
Estimated repair costs for P0138 in EUR, compiled across makes. Expect Mercedes-Benz parts and labour towards the upper end of each range.
| Repair | Parts Cost | Labor Cost | Total Estimate | DIY Difficulty |
|---|---|---|---|---|
| Replace downstream O2 sensor (Bank 1, Sensor 2) | €40–€180 | €50–€130 | €90–€310 | Moderate |
| Repair shorted O2 sensor wiring | €5–€30 | €60–€150 | €65–€180 | Moderate |
| Diagnose and fix rich condition (injector, fuel pressure) | €30–€400 | €80–€250 | €110–€650 | Professional |
Prices estimated as of October 2026. Costs vary by region, vehicle, and shop.
Codes related to P0138
These codes are often seen alongside P0138 or point to related issues, on any make:
Looking for the full P0138 reference (all makes, full diagnosis flow, complete repair cost matrix)? See the main P0138 guide