Description

DATA TROUBLE CODE: P0171

SYSTEM: System too Lean (Fuel Trim)

CIRCUIT DESCRIPTION
"Fuel trim" refers to the feedback compensation value compared against the basic injection time. Fuel trim includes short-term fuel trim and long-term fuel trim.
"Short-term fuel trim" is the short-term fuel compensation used to maintain the air-fuel ratio at its ideal theoretical value. The signal from the Heated Oxygen Sensor indicates whether the air-fuel ratio is RICH or LEAN compared to the ideal theoretical value, triggering a reduction in fuel volume if the air-fuel ratio is rich, and an increase in fuel volume if it is lean.
"Long-term fuel trim" is overall fuel compensation carried out long-term to compensate for continual deviation of the short-term fuel trim from the central value due to individual engine differences, wear over time and changes in the usage environment.
If both the short-term fuel trim and long-term fuel trim are LEAN or RICH beyond a certain value, it is detected as a malfunction and the MIL lights up.

DIAGNOSTIC TROUBLE CODE DETECTING CONDITION
- When the air fuel ratio feedback is stable after engine warming up, the fuel trim is considerably in error on the RICH side (2 trip detection logic).

TROUBLE AREA
- Air intake (hose loose).
- Fuel line pressure.
- Injector blockage.
- Heated Oxygen Sensor malfunction.
- Mass Air Flow Meter.
- Engine Coolant Temperature Sensor.

NOTE: When Data Trouble Code P0171 is recorded, the actual air-fuel ratio is on the LEAN side. When Data Trouble Code P0172 is recorded, the actual air-fuel ratio is on the RICH side.
Fuel trim applies separately to bank 1 and bank 2, 50 the Engine Control Module lights up the Malfunction Indicator Lamp if a problem occurs with either bank.
You can tell which bank is malfunctioning by looking at the short-term fuel trim and long-term fuel trim, thus allowing you to focus your inspection.
If the total of the short-term fuel trim value and long-term fuel trim value for each bank is within ±25 %, the system is functioning normally.