P0171: Mixture too lean (bank 1)

What P0171 means

A lean mixture means there is more air than the injected fuel calls for. The ECU tries to compensate by adding fuel, and this code is set when it reaches the limit of what it can correct. On inline engines, bank 1 is the whole engine.

Most common causes

• Cracked or loose vacuum hose, or split intake hose after the air flow meter
• Dirty air flow meter reading less air than is really entering
• Leaking intake manifold gasket or crankcase ventilation (PCV) valve
• Blocked fuel filter or weak fuel pump
• Dirty or partly clogged injectors
• Exhaust leak ahead of the oxygen sensor

Symptoms

• Engine warning light on
• Unsteady or hunting idle
• Hesitation or lack of power when accelerating
• Hissing sound in the engine bay (if there is an air leak)
• Hard starting

Can you drive with P0171?

With care You can drive, but a lean mixture over time can cause misfires and make the engine and catalytic converter run hotter. Have it checked soon.

How to check it

Watch the long term fuel trim for bank 1 in live data: if it is very high at idle and falls as revs rise, an air leak is most likely; if it also rises under load, it points to a lack of fuel. With the engine running, listen for a hiss.

How to read and clear P0171 with Final OBD

Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0171 with this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

• P0158 Oxygen sensor (bank 2, sensor 2): voltage too high
• P0160 Oxygen sensor (bank 2, sensor 2): no activity
• P0161 Oxygen sensor heater (bank 2, sensor 2): circuit fault
• P0170 Fuel trim out of limits (bank 1)
• P0172 System running too rich (bank 1)
• P0173 Fuel trim malfunction (bank 2)
• P0174 System running too lean (bank 2)
• P0175 System running too rich (bank 2)
• P0100 Mass or volume air flow sensor circuit fault
• P0101 Air flow sensor range or performance problem
• P0102 Air flow sensor circuit low input
• P0103 Air flow sensor circuit high input

P0171 is a generic code, so it means the same on any make, for example on Audi, Dacia, Fiat, Kia, Opel / Vauxhall, Toyota. See what Final OBD does on your car in makes and models.