What does code P0171 mean on a Mercedes-Benz and is it safe to drive?
P0171: System Too Lean (Bank 1). Here's what it means on your Mercedes-Benz, what usually causes it, what repairs typically cost, and how safe it is to keep driving.
What this code means
The engine is getting too much air relative to fuel. The O2 sensor sees excess oxygen in the exhaust and the PCM is adding more fuel than normal to compensate.
Common causes for your Mercedes-Benz
- Dirty or faulty mass airflow (MAF) sensor
- Vacuum leaks (PCV hose, intake gasket, brake booster)
- Weak fuel pump or clogged fuel filter
- Leaking or stuck open fuel injectors (less common)
Symptoms
- Check Engine Light
- Rough idle, hesitation, lack of power
- Possible spark knock under load
- Poor fuel economy
Typical fixes
- Clean or replace MAF sensor
- Locate and seal vacuum leaks
- Replace fuel filter / test fuel pressure
- Smoke test intake system
Repair cost range
For a Mercedes-Benz, repairs for P0171 often fall between $50 and $500, depending on the cause and whether you use OEM or aftermarket parts. Labor rates vary by location.
Known issues on Mercedes-Benz
Mercedes P0171 on M274 2.0T engines is frequently an SAI (Secondary Air Injection) pump failure—the SAI pump injects air into the exhaust manifold during cold starts to help catalytic converter light-off. When the SAI pump or check valve fails, the ECU sees a lean condition during startup and sets P0171. On M272/M276 V6 engines, the intake manifold secondary air passages clog with carbon deposits. On M271 1.8T Kompressor, the charge air hose from the supercharger to the throttle body splits under boost. Always perform a smoke test on M-B engines before replacing sensors.
Most commonly affected models