DTC P0171 Or DTC P0172: Condition

WARNING: This page is about a different car, the 2002 Suzuki Esteem. However, it is still accessible from the selected car via links, so may be relevant.

DTC is set when engine is in closed loop and air/fuel ratio is too lean (short term fuel trim is more than 20 percent or total fuel trim is more than 30 percent). DTC is also set when engine is in closed loop and air/fuel ration is to rich (short term fuel trim is less than -20 percent or total fuel trim is less than -30 percent). Total fuel trim is calculated by adding short and long fuel trim. ECM continuously monitors this circuit. MIL is illuminated when ECM detects malfunction during 2 consecutive drive cycles. Possible causes are:

Intake vacuum leaks.
Exhaust system leaks.
HO2S No. 1 circuit malfunction.
Incorrect fuel pressure.
Fuel injector malfunction.
Manifold Absolute Pressure (MAP) sensor malfunction.
Engine Coolant Temperature (ECT) sensor malfunction.
Intake Air Temperature (IAT) sensor malfunction.
Throttle Position (TP) sensor malfunction.
EVAP control system malfunction.
Positive Crankcase Ventilation (PCV) valve malfunction.