Circuit Description
Fuel trim is related to feedback compensation value, not to 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 its 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 in long-term to compensate for a continual deviation of short-term fuel trim from central value, due to individual engine differences, wear over time and changes in using environment. If both short-term fuel trim and long-term fuel trim are rich or lean beyond a certain value, it is detected as a malfunction and the MIL illuminates.
DTC P0171 and P0174 set when fuel trim is lean beyond a certain value (2 trip detection logic). Possible causes are:
- Air induction system malfunction.
- Fuel injector restricted.
- Defective Mass Airflow (MAF) meter.
- Defective Engine Coolant Temperature (ECT) sensor.
- Improper fuel pressure.
- Exhaust system leak.
- Open or short in heated oxygen sensor circuit (bank 1 or 2, sensor 1).
- Defective heated oxygen sensor (bank 1 or 2, sensor 1).
- Defective heated oxygen sensor heater (bank 1 or 2, sensor 1).
- Defective EFI relay.
- Faulty ECM.
- Faulty PCV valve, hose or connection.
DTCs P0172 and P0175 set when fuel trim is rich beyond a certain value (2 trip detection logic). Possible causes are:
- Fuel injector restricted or leaking.
- Defective Mass Airflow (MAF) meter.
- Defective Engine Coolant Temperature (ECT) sensor.
- Ignition system malfunction.
- Improper fuel pressure.
- Exhaust system leak.
- Open or short in heated oxygen sensor circuit (bank 1 or 2, sensor 1).
- Defective heated oxygen sensor (bank 1 or 2, sensor 1).
- Faulty ECM.