Trouble Code:
TOYOTA P0171 System Too Lean Bank 1
Likely Cause:
- The MAF (Mass Air Flow) Sensor is dirty or faulty
- There could be a vacuum leak downstream of the MAF sensor
- Possible cracked vacuum or PCV line/connection
- Faulty or stuck open PCV valve
- Failed or faulty oxygen sensor (bank 1, sensor 1)
- Sticking/plugged or failed fuel injector
- Low fuel pressure (possible plugged/dirty fuel filter!)Exhaust leak between engine and first oxygen sensor
Subsystem:
You will more than likely not notice any drivability problems, although there may be symptoms such as:
- A lack of power
- Detonation (spark knock)
- Rough idle Hesitation/surge on acceleration.
Notes:
The use of “oiled” air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
What is the TOYOTA P0171 code mean?
Basically this means that an oxygen sensor in bank 1 has detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1. The P0171 is one of the more common trouble codes.
This code is triggered by the first downstream (front) O2 sensor. The sensor provides a reading of the air:fuel ratio leaving the engine’s cylinders, and the vehicles powertrain/engine control module (PCM/ECM) uses that reading and adjusts to keep the engine running at that optimum ratio of 14.7:1. If something is not right and the PCM cannot maintain the 14.7:1 ratio, but rather there is too much air, it triggers this code. You’ll want to also read our article on short and long term fuel trims to help understand the operation of the engine.
How to reset the code TOYOTA P0171 code?
If the air-fuel mixture becomes too lean and the ECM can no longer compensate, the engine computer (ECM) illuminates the “Check Engine” light (MIL) and sets the trouble code P0171 – System Too Lean for Bank 1 and (or) P0174 – System Too Lean Bank 2, depending on which cylinder bank is affected. The ECM also stores the freeze frame of the parameters (engine temperature, vehicle speed, fuel trim readings, etc.) at the time when the code was set. Typically the code P0171 is set when either short term fuel trim (STFT) or long term fuel trim (LTFT) adjustment exceeds a certain value (usually +25%). Read more how the air/fuel ratio control works below.