Circuit Description

Fuel trim refers to feedback compensation value compared against basic injection time. Fuel trim includes short-term fuel trim and long-term fuel trim. Short-term fuel trim is short-term fuel compensation used to maintain air-fuel ratio at it's ideal theoretical value. Signal from heated oxygen sensor indicates whether air-fuel ratio is RICH or LEAN compared to ideal theoretical value, triggering a reduction in fuel volume if 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 short-term fuel trim from central value due to individual engine differences, wear over time and changes in usage environment. If both short-term fuel trim and long-term fuel trim are LEAN or RICH beyond a certain value, it is detected as a malfunction and Malfunction Indicator Light (MIL) lights up.

Detecting condition of DTCs is when air-fuel ratio feedback is stable after engine warming up, fuel trim is considerably in error on RICH (P0171) or LEAN (P0172) side (2 trip detection logic). If total of short-term fuel trim value and long-term fuel trim value is within plus or minus 25%, system is functioning normally.