Circuit Description

WARNING: This page is about a different car, the 2003 Lexus ES 300. However, it is still accessible from the selected car via links, so may be relevant.

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 air/fuel ratio sensor is approximately proportional to existing air-fuel ratio. Comparing it with ideal theoretical value, ECM reduces fuel volume immediately if air-fuel ratio is rich and increases fuel volume if it is lean. Long-term fuel trim compensates for deviation from central value of short-term fuel trim stored up by each engine tolerance, and deviation from central value due to passage of time and changes of environment. If both short-term fuel trim and long-term fuel trim exceed a certain value, it is detected as a malfunction and MIL lights up.

DTC P0171 and DTC P0174 are set when air/fuel ratio is stable after warming up engine and fuel trim is considerably in error on lean side (2 trip detection logic). Possible causes are:

Air induction system.
Injector blockage.
Mass air flow meter.
Engine coolant temperature sensor.
Fuel pressure
Gas leakage in exhaust system.
Open or short in A/F sensor circuit.
A/F sensor malfunction (bank 1, 2 sensor 1).
Air/fuel ratio sensor heater and relay circuit.
PCV hose connection.
PCV hose.

DTC P0172 and DTC P0175 are set when air/fuel ratio feedback is stable after warming up engine, and fuel trim is considerably in error on rich side (2 trip logic). Possible causes are:

Injector leak or blockage.
Mass air flow meter.
Engine coolant temperature sensor.
Ignition system.
Fuel line pressure.
Gas leakage in exhaust system.
Open or short in A/F sensor circuit (bank 1, 2 sensor 1).
A/F sensor malfunction.
Air/fuel ratio sensor heater relay.
Air/fuel ratio sensor heater and relay circuit.