DTC P0170 (FUEL): FUEL TRIM MALFUNCTION Subaru Forester SF

1. Check for the following items in exhaust system: Looseness of installation components. Damage to parts (cracks, holes, etc.). Looseness or opening of parts between front and rear O2S. Replace or repair defective parts. If no defects are found, go to next step.
2. Check for the following items in air intake system: Looseness of installation components. Damage to parts (cracks, holes, etc.). Disconnection of hose on air intake system. Replace or repair defective parts. If no defects, go to next step.
3. On Forester, disconnect fuel pump relay (located by left kick panel) under dashboard at far left side. Start and operate engine until it stalls. After engine stalls, crank engine for 5-7 seconds to ensure reduced fuel pressure. Turn ignition off. Remove fuel filler cap. Reconnect fuel pump relay. Go to step 5). WARNING: Release fuel pressure before removing fuel lines.
4. On Impreza and Legacy, remove fuel pump access cover located at right rear of trunk (sedan), or luggage compartment floor (wagon). Disconnect connector from fuel tank. Start and operate engine until it stalls. After engine stalls, crank engine for 5-7 seconds to ensure reduced fuel pressure. Turn ignition off. Remove fuel filler cap. Reconnect fuel tank connector. Go to next step. NOTE: During test, if fuel pressure does not increase, squeeze fuel return hose 2-3 times and recheck fuel pressure.
5. On all models, disconnect fuel delivery hose from fuel filter and connect a fuel pressure gauge between filter and delivery hose. Install fuel filler cap. Start engine and idle with transmission in Park or Neutral. Check fuel pressure with pressure regulator hose disconnected from intake manifold. See «FUEL PRESSURE SPECIFICATIONS»(ref-26343-S35176079412001010500000) table. If fuel pressure is within specifications, go to next step. If fuel pressure is too high, check for clogged fuel return line or bent hose. If fuel pressure is too low, check for improper fuel pump discharge or clogged fuel supply line. FUEL PRESSURE SPECIFICATIONS Model Vacuum Line psi (kg/cm 2 ) All models Connected 23-30 (1.6-2.1) " Disconnected 33-40 (2.3-2.8)
6. Check fuel pressure with pressure regulator vacuum hose connected. See «FUEL PRESSURE SPECIFICATIONS»(ref-26343-S35176079412001010500000) table. If fuel pressure is within specifications, go to next step. If fuel pressure is too high, check for faulty pressure regulator, clogged fuel return line or bent hose. If fuel pressure is too low, check for faulty pressure regulator, improper fuel pump discharge or clogged fuel supply line. WARNING: Release fuel pressure before removing fuel pressure test gauge. See step 3) for Forester, and step 4) for Impreza and Legacy.
7. Remove fuel pressure test gauge. Start engine. Check for leaks. Turn ignition off. Connect scan tool. Start and warm engine to normal operating temperature. Observe coolant temperature display. If temperature reading is more than 140°F (60°C), go to next step. If temperature reading is less than 140°F (60°C), replace Engine Coolant Temperature (ECT) sensor.
8. Ensure all accessories are off. Set scan tool to display MAF sensor signal. See «MASS AIRFLOW SENSOR VALUES»(ref-26343-S06889964322001010500000) table. If the mass airflow value is not as specified, replace MAF sensor and retest system. If mass airflow value is as specified, replace PCM.

MASS AIRFLOW SENSOR VALUES