Circuit Description

WARNING: This page is about a different car, the 2003 Lexus SC 430. However, it is still accessible from the selected car via links, so may be relevant.

Fuel trim refers to feedback compensation value compared against basic injection time. Fuel trim includes short-term and long-term fuel trim. The 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 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 carried out in long-term to compensate for a continual deviation of the short-term fuel trim from the central value, due to individual engine differences, wear overtime and changes in the using environment. If both the short-term fuel trim and the long-term fuel trim are LEAN or RICH beyond a certain value, it is detected as a malfunction and the MIL (CHECK ENGINE) lights up.

DTC P0171 or P0174 is set when fuel trim is lean beyond a certain value. Possible causes are:

Air intake hose loose.
Fuel injector restricted.
Faulty Mass Airflow (MAF) sensor.
Faulty Engine Coolant Temperature (ECT) sensor.
Improper fuel line pressure.
Exhaust system leak.
Open or short in heated oxygen sensor circuit.
Faulty heated oxygen sensor.
PCV hose connection.
Faulty ECM.

DTC P0172 or P0175 is set when fuel trim is rich beyond a certain value. Possible causes are:

Fuel injector restricted or leaking.
Faulty Mass Airflow (MAF) sensor.
Faulty Engine Coolant Temperature (ECT) sensor.
Ignition system malfunction.
Improper fuel line pressure.
Exhaust system leak.
Open or short in heated oxygen sensor circuit.
Faulty heated oxygen sensor.
Faulty ECM.