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Home >> Subaru >> 2003 >> Outback Limited, 4D Sedan >> Repair and Diagnosis (Single Page) >> Engine Performance >> System >> Engine Controls - Self-Diagnostics - 2.5L >> Diagnostic Tests >> DTC P0172: Fuel Trim Malfunction (Too Rich - Forester & IMPREZA) >> Diagnostic Procedure

Diagnostic Procedure

  1. Perform basic diagnostic procedure. See BASIC DIAGNOSTIC PROCEDURE . Check for loose bolts and/or holes in exhaust system. Repair as required. If exhaust system is sealed and has no loose components, go to next step.
  2. Check for holes, loose bolts and/or disconnected hoses in air intake system. Repair as required. If air intake system is sealed and has no loose or disconnected components, go to next step.
  3. Check purge control solenoid valve for problems. If purge control solenoid valve is okay, go to next step. If purge control solenoid valve is not okay, repair as required.
    4. WARNING: Release fuel pressure before removing fuel lines. On Forester models, see FUEL SYSTEM PRESSURE RELEASE in REMOVAL & INSTALLATION - FORESTER article. On Impreza and Outback Sport models, see FUEL SYSTEM PRESSURE RELEASE in REMOVAL & INSTALLATION - IMPREZA & OUTBACK SPORT article.
    NOTE: During test, if fuel pressure does not increase, squeeze fuel return hose 2-3 times and recheck fuel pressure.
  4. 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.
  5. 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. On Forester models, see FUEL SYSTEM PRESSURE RELEASE in REMOVAL & INSTALLATION - FORESTER article. On Impreza and Outback Sport models, see FUEL SYSTEM PRESSURE RELEASE in REMOVAL & INSTALLATION - IMPREZA & OUTBACK SPORT article.
  6. Remove fuel pressure test gauge and reconnect fuel delivery hose. Start engine. Check for leaks. Turn ignition off. Start and warm engine to normal operating temperature. Using scan tool, observe coolant temperature. If coolant temperature is 158-212°F (70°-100°C), go to next step. If coolant temperature is not 158°F -212°F (70°C -100°C), replace engine coolant temperature sensor.
  7. Start engine. Operate engine until coolant temperature is greater than 140°F (60°C). Place transaxle in Neutral. Ensure all accessories and A/C are off. Using scan tool, read intake manifold absolute pressure. If pressure is 7.09-12.20 in. Hg with engine idling and 21.65-31.50 in. Hg with ignition on (engine off), contact authorized Subaru dealership because probable cause is deterioration of multiple parts. If pressure is not 7.09-12.20 in. Hg with engine idling and 21.65-31.50 in. Hg with ignition on (engine off), replace intake manifold pressure sensor. Clear memory and perform verification procedure. See CLEARING DIAGNOSTIC TROUBLE CODES  and VERIFICATION PROCEDURE  .
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