Subaru Impreza and Forester OBD-II Diagnostics: EJ20/EJ25, WRX P0299, HGF, AVCS

Subaru Impreza, Forester, Outback and Legacy are vehicles with a horizontally-opposed (Boxer) engine and Symmetrical All-Wheel Drive. The EJ engine series is well-known in Europe but has specific weak points that diagnose well via OBD-II.

EJ20 / EJ25 2.0–2.5 (Impreza, Forester, Outback): Naturally Aspirated Boxer

P0301–P0304 — Misfires: Head Gasket Failure

HGF (Head Gasket Failure) — the most notorious EJ20/EJ25 issue:

EJ25 (2.5 NA) after 120,000 km is prone to head gasket leakage between the coolant passage and oil passage, or into the combustion chamber:

OBD-II symptoms of HGF:

• P0301–P0304: misfires from coolant entering cylinders
• Coolant Temperature doesn’t stabilise, climbs slowly
• Long Term Fuel Trim B1/B2 > +8% from lean mixture (coolant burning)

Live Data HGF check:

• Coolant Temp after 10 min driving: if > 100°C → overheating (coolant loss)
• MAF Reading vs normal at idle: drop → EGR or HGF toward intake manifold
• Gas analyser: CO > 0.5% on warm engine → HGF or misfires

P0420 / P0430 — Catalyst EJ20/EJ25

Naturally aspirated EJ20/EJ25 after 150,000 km:

• O2 B1S2 or O2 B2S2 actively oscillating → catalyst has lost efficiency
• Subaru has 2 catalysts (one per bank) + a centre catalyst
• P0420 and P0430 simultaneously → centre cat or both pre-cats

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EJ257 2.5T WRX STI: Turbocharged Boxer

P0299 — Underboost EJ257

WRX STI with IHI VF52/VF53 turbocharger:

• Boost Pressure Target vs Actual at 4,000 RPM: > 30 kPa → boost controller or wastegate
• Wastegate Duty Cycle > 90% in all modes → check wastegate actuator pressure

Known EJ257 issue: Wastegate Swing Valve — tears at ≈60,000 km on high-boost tuning.

P0234 — Overboost WRX

P0234 = boost above maximum limit:

• Boost Pressure Actual > 1.4 bar (EJ257 normal 1.2–1.25) → wastegate stuck closed

P0011 / P0021 — AVCS EJ257

AVCS (Active Valve Control System) — Subaru VVT:

• P0011 — B1 intake camshaft phase: over-advanced
• P0021 — B2 intake camshaft phase

Live Data:

• AVCS Target vs Actual B1/B2: > 5° difference → solenoid or oil pressure issue
• EJ257 AVCS solenoids sensitive to oil quality → mandatory oil change every 5,000 km

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FB20 / FB25 (Forester SJ/SK, Impreza GP/GK): Modern Boxer

P0011 — AVCS FB20

FB20 with AVCS on both intake and exhaust:

• AVCS Intake Actual vs Target B1/B2 > 8° → solenoid fouled
• Common problem with infrequent oil changes

P2187 / P2189 — Lean Mixture FB20

FB20 and some FB25 — direct injection (DIT) or mixed:

• STFT B1/B2 > +8% → carbon or PCV leak
• Subaru FB has known PCV valve issues on FB20DIT

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FA20DIT (WRX VA): Direct-Injection Turbo

P0299 — FA20DIT Underboost

FA20DIT (WRX 2015+) with IHI VF52:

• Boost Target vs Actual > 25 kPa at 3,000 RPM → similar issue to EJ257
• Charge Air Temp after IC > 55°C → intercooler or boost leak

P2187 — GDI Carbon FA20DIT

FA20DIT — direct injection without port injectors:

• STFT B1/B2 > +10% → carbon buildup (common to all GDI engines)
• Walnut Blasting after 80,000 km

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Subaru AWD: Symmetrical All-Wheel Drive

P0771 — AWD Slip (Automatic Gearbox)

On Subaru with automatic (5EAT / Lineartronic CVT):

• P0771 = FWD/AWD Clutch Slip: viscous coupling or torque transfer issue
• AWD Torque Distribution Front/Rear: should be ≈50/50 on flat road

Lineartronic CVT (TR580 / TR690)

Subaru Lineartronic CVT — similar principle to Jatco:

• CVT Belt Temperature > 120°C → towing or heavy traffic
• CVT Line Pressure < 6 bar at 50 km/h → pump or belt wear

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OBD-II Specifics for Subaru

Subaru uses ISO 15765 (CAN) + Select Monitor III (SSM3):

• Mode 01 PIDs: standard
• AVCS, DIT, AWD extended: only via SSM3 or Subaru-compatible scanner (e.g., FORSCAN with Subaru module)

Important extended PIDs:

• AVCS Oil Pressure B1/B2 — critical for WRX STI
• Knock Correction B1/B2 — EJ257 detonates on low-octane fuel
• WRX Wastegate Duty Cycle — SSM3 or equivalent

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Summary: Subaru Impreza / Forester

Model	Engine	Top Codes
Impreza/Forester	EJ25 NA	P0301–P0304 HGF, P0420/P0430
WRX STI	EJ257 Turbo	P0299 boost, P0011 AVCS
Forester SJ/SK	FB20/FB25	P0011 AVCS, P2187 lean
WRX VA	FA20DIT	P0299, P2187 GDI
All AWD	5EAT/CVT	P0771 AWD, CVT pressure

EJ25 head gasket failure is the most expensive repair in the range: at the first symptoms (P0301 + unstable Coolant Temp), perform a cylinder pressure test and CO emissions test before spending thousands on engine work.