Parts for your 2015 Subaru Impreza-Alternator

2015 Subaru Impreza alternator — what it does, when to replace it, and how to look after it

Based on Subaru factory service information for the 2012–2016 Impreza with the FB20 engine, plus the owner’s manual and OE parts catalogues, the 2015 Subaru Impreza is fitted with an engine-driven alternator (smart charging type). It’s a core part of the charging system and absolutely relevant to this model.

On a 2015 Impreza, the alternator keeps the 12V battery topped up and powers the car’s electrical gear while the engine’s running — headlights, blower fan, heated seats, wipers, the lot. Subaru uses an ECU-controlled (smart) alternator that varies output to suit load and driving conditions, which helps fuel economy. So, voltage can move around more than old-school units, and that’s normal.

As part of routine servicing, it pays to give the alternator and its drive belt a once-over. A healthy unit will typically keep system voltage around 13.5–14.8 V under load, though it may dip into the low 12s during decel or light load with smart charging. If the battery light flickers, lights dim at idle, or there’s a whining/grinding noise from the front of the engine, it’s time for testing.

• Check the drive belt for cracks, glazing, or slack, and listen for pulley noise.
• Test battery health, a weak battery can make the alternator work overtime.
• Inspect grounds and battery terminals for corrosion — clean and tighten.
• Measure voltage at the battery posts with a quality multimeter.

Replacement is straightforward for a pro: disconnect the battery, remove the belt, swap the unit, refit and tension the belt, then verify charging voltage and that the battery light’s out. On smart systems, never disconnect the battery with the engine running. If equipped with a decoupler pulley, confirm it freewheels correctly, replace if it’s noisy or seized. After any battery or alternator work, let the engine idle for a short drive cycle so the ECU relearns trims — no special programming is typically needed on this model.

There’s no fixed kilometre interval for alternator replacement, Subaru recommends inspection at regular service visits. With quality parts and good battery care, many Impreza alternators run well past 150,000 km. Keeping the belt in shape and the electrical connections clean is the simplest way to avoid dramas and keep the charging system sweet.

Popular questions

What are common symptoms of a failing 2015 Impreza alternator?
Typical signs include a red battery/charge warning light, dim or flickering headlights, slow or erratic electrics, whining or grinding from the alternator area, and a flat battery after short trips. If multiple warning lights appear at once, voltage may be dropping — get it tested promptly.

What charging voltage should be seen on this model?
With the engine running, expect roughly 13.5–14.8 V under normal load. Because it’s a smart charging setup, you may see brief readings in the low 12s during decel or light load and higher 14s after a cold start. Measure at the battery posts with a good multimeter and check under different loads (lights, rear demister, fan) to confirm stability.

Is it safe to drive with the battery light on?
Only short distances to a workshop if necessary. If the alternator isn’t charging, the car will eventually run out of electrical power and stall. Switch off non-essential accessories and get the charging system tested as soon as possible to avoid being stranded.