DTC P0170: Fuel Trim Malfunction - Forester, IMPREZA 2.5L & Legacy & Outback 2.5L A/T

Perform basic diagnostic procedure. See BASIC DIAGNOSTIC PROCEDURE . Check for faulty exhaust system components, cracks and/or leaks. Replace or repair defective parts. If no defects are present, go to next step.
Check air intake system components for cracks and/or leaks. Replace or repair defective parts. If no defects are present, go to next step.
WARNING: Release fuel pressure before removing fuel lines. See FUEL SYSTEM PRESSURE RELEASE in BASIC DIAGNOSTIC PROCEDURES.

NOTE: During test, if fuel pressure does not increase, squeeze fuel return hose 2-3 times and recheck fuel pressure.
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 transaxle in Neutral. Check fuel pressure with pressure regulator hose disconnected from intake manifold. See FUEL SYSTEM in SERVICE & ADJUSTMENT SPECIFICATIONS article. If fuel pressure is within specifications, go to next step. If fuel pressure is too high, check for blocked fuel return line or bent hose. If fuel pressure is too low, check for improper fuel pump discharge or blocked fuel supply line.
Check fuel pressure with pressure regulator vacuum hose connected. See FUEL SYSTEM in SERVICE & ADJUSTMENT SPECIFICATIONS article. If fuel pressure is within specifications, go to next step. If fuel pressure is too high, check for faulty pressure regulator, blocked fuel return line or bent hose. If fuel pressure is too low, check for faulty pressure regulator, improper fuel pump discharge or blocked fuel supply line.
WARNING: Release fuel pressure before removing fuel pressure test gauge.
Remove fuel pressure test gauge. Start engine. Check for leaks. Turn ignition off. Connect scan tool. Start and warm engine to normal operating temperature. Using scan tool. observe coolant temperature. If temperature is between 158°F (70°C) and 212°F (100°C), go to next step. If temperature is not between 158°F (70°C) and 212°F (100°C), replace engine coolant temperature sensor.
Start and warm engine to temperature greater than 140°F (60°C). Place transaxle in Neutral or Park. Ensure A/C and all accessories are off. Set scan tool to display intake manifold pressure signal. If manifold vacuum is not 7.09-12.2 in. Hg at idle and 21.65-31.50 in. Hg with ignition on, replace intake air temperature and pressure sensor. If manifold vacuum is 7.09-12.2 in. Hg at idle and 21.65-31.50 in. Hg with ignition on, contact authorized Subaru dealership because probable cause is deterioration of multiple parts. Clear memory and perform verification procedure. See CLEARING DIAGNOSTIC TROUBLE CODES and VERIFICATION PROCEDURE .