DTC P0170: Fuel Trim Malfunction - Except IMPREZA & Legacy 2.2L (CALIFORNIA Emissions)

Check for the following problems in exhaust system:
- Looseness of installed components.
- Damage to parts (cracks, holes, etc.).
  Replace or repair defective parts. If no defects are found, go to next step.
Check for the following problems in air intake system:
- Looseness of installed 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.
WARNING: Release fuel pressure before removing fuel lines.

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 transmission in Park or 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. 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.

Ensure all accessories are off. Set scan tool to display MAF sensor signal. See MASS AIRFLOW SENSOR VALUES table. If mass airflow value is not within specifications, replace MAF sensor. If mass airflow value is within specifications, replace ECM.

MASS AIRFLOW SENSOR VALUES

Engine Speed	Specified Value (g/sec)
Idling	2.2-4.2
2500 RPM	8.6-14.5