Circuit Description
The fuel trim is related to the feedback compensation value, not to the basic injection time. The fuel trim includes the short-term fuel trim and the long-term fuel trim.
The short-term fuel trim is the short-term fuel compensation used to maintain the air-fuel ratio at stoichiometric air-fuel ratio. The signal from the heated oxygen sensor indicates whether the air-fuel ratio is RICH or LEAN compared to the stoichiometric air-fuel ratio. This variance triggers a reduction in the fuel volume if the air-fuel ratio is RICH, and an increase in the fuel volume if it is LEAN.
The long-term fuel trim is the overall fuel compensation and carried out in "long-term" to compensate a continual deviation from the theoretical central value by the "short-term" fuel trim, due to individual engine's specifications and engine deterioration with time.
If both short-term and long-term fuel trims are LEAN or RICH beyond a certain value, it is detected as a malfunction and the MIL is illuminated.
HINT:
- When DTC P0171 is recorded, the actual air-fuel ratio is on the LEAN side. When DTC P0172 is recorded, the actual air-fuel ratio is on the RICH side.
- If the vehicle runs out of fuel, the air-fuel ratio is LEAN and DTC P0171 may be recorded. The MIL then comes on.
- If the total of the short-term fuel trim value and long-term fuel trim value is within +/- 35 % (engine coolant temperature is more than 75 °C (167 °F)), the system is functioning normally.