Subaru Fault Code

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P0171

Surging on Throttle / Positive Fuel Trim

System

Fuel / Induction

Symptom

Surging on Throttle / Positive Fuel Trim

Diagnosis & fix steps

LOGIC: System Too Lean (Bank 1 LTFT > 25%). ACTION: Smoke test the intake tract downstream of the Mass Air Flow (MAF) sensor. Inspect the accordion turbo inlet pipe or PCV breather hoses for cracks. Clean the MAF sensor hot wire with dedicated cleaner.

Red Seal field insights

Cracked Turbo Inlet: Boxer engine layouts place the flexible rubber intake inlet pipe under the intake manifold; thermal fatigue cracks the inlet elbow at the turbo clamp, leaking unmetered air.

Same code keeps coming back?

That's a program problem, not a part problem. Book a 30-minute uptime review with Grant, a Red Seal Heavy Duty Technician.

Book an uptime review

Read before you touch anything

  • This is general reference information, not a repair manual. It is for trained, qualified technicians.
  • Fault codes, meanings and procedures vary by model, serial number and software version. Confirm the code and the procedure against the manufacturer's service manual before acting.
  • Follow the OEM service manual, torque specs and your site's safety procedures: lockout/tagout, block raised equipment, relieve hydraulic, fuel and air pressure, and let hot systems cool.
  • You use this information at your own risk. Adventure Mechanical and AM Fleet Integrity accept no liability for damage, injury, downtime or loss arising from its use.
  • Not affiliated with or endorsed by any manufacturer. Brand names are used only to identify the equipment a code applies to.

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