Subaru Forester & Impreza Alternator Guide (2004-2018): Whining Noises, Misfire Codes, and Replacement

Critical Compatibility Warnings: Read This First!

The alternators for the 2006-2018 Subaru Forester and 2004-2012 Subaru Impreza are NOT one-size-fits-all. Fitment depends entirely on the model year, generation, and engine type (specifically, non-turbo vs. turbo). Using the wrong alternator can lead to charging problems, damage to your vehicle's electrical system, or it may not physically bolt on. The initial buyer warnings you may have seen are often conflicting because they apply to different, specific versions of these cars.

Symptoms of a Failing Subaru Alternator

When an alternator begins to fail, it gives several clear warning signs. Ignoring them will eventually lead to a dead battery and a car that won't start.

• Whining or Grinding Noise: The most common symptom is a high-pitched whine or a grinding sound coming from the engine bay that rises and falls with engine RPM. This is the sound of the internal bearings wearing out. A squealing sound, 🎬 Watch: Diagnose engine whining noises and replace drive belts by contrast, is more often a loose or worn drive belt.
• Battery Warning Light: A dashboard light shaped like a battery is a direct indicator of a charging system problem. This means the battery is not being recharged by the alternator.
• Dim or Flickering Lights: Headlights and dashboard lights may appear dim, yellowish, or flicker, especially when the engine is at idle. They might brighten when you rev the engine.
• Strange Electrical Problems: You might notice power windows moving slowly, the radio cutting out, or other electrical accessories acting erratically as they are starved for consistent power.
• Dead Battery or Stalling: A failing alternator can't keep the battery charged, leading to a no-start condition. A classic test is to jump-start the car; if it dies immediately after the jumper cables are removed, the alternator is almost certainly bad.
• Burning Smell: A smell like burnt rubber or hot electrical wires can mean the alternator is overheating or its belt is slipping and burning.

How to Diagnose a Bad Alternator in 5 Minutes

You can definitively test your alternator with a simple tool called a multimeter. It's an inexpensive device that measures voltage.

1. Test Battery Voltage (Engine Off): Set your multimeter to DC volts. With the engine off, touch the red probe to the positive (+) battery terminal and the black probe to the negative (-) terminal. A healthy, fully charged battery should read between 12.4 and 12.7 volts.
2. Test Charging Voltage (Engine Running): Start the engine and let it idle. Measure the voltage across the battery terminals again. A healthy alternator should be putting out between 14.2 and 14.7 volts to charge the battery.

If the reading is below 13 volts or close to the initial battery voltage, the alternator is not charging. If the reading is significantly higher (e.g., over 15 volts), the internal voltage regulator has failed, which can overcharge and damage your battery.

Known Issues, TSBs, and Recalls

While there are no direct, large-scale recalls for these alternators, several important service bulletins and known issues exist.

TSB for False Misfire Codes (P0300) on 2014-2016 Forester

A critical issue affects some 2014-2016 Foresters with the 2.5L engine. According to Subaru Technical Service Bulletin 07-112-16R, current fluctuations from the original alternator can trick the engine's computer into setting a P0300 (Random/Multiple Cylinder Misfire) code, even when there is no actual engine misfire. The official fix is to replace both the alternator AND the accessory belt tensioner with updated parts as a set. Simply clearing the code or replacing spark plugs will not solve the problem.

Early Model Charging Cut-Out Issue (2006-2008 Forester)

Some earlier models (primarily 2001-2008 Foresters) feature an ECU function designed to improve fuel economy by cutting the alternator's charging output under certain low-load conditions. This can sometimes lead to the battery not getting a full charge and eventually going flat. While less common, it's a known quirk that can be misdiagnosed as a failed alternator.

Widespread Battery Drain Complaints

Many Subaru models from this era were part of a class-action settlement related to battery drain issues. A dead battery is not always the alternator's fault. If your alternator tests good but your battery repeatedly dies, the vehicle may have a parasitic draw (an electrical component that stays on when it shouldn't).

Buying a Used OEM Alternator: What to Look For

For many owners, a quality used OEM (Original Equipment Manufacturer) alternator is a smart choice, offering better reliability than a cheap aftermarket part at a fraction of the cost of a new one from the dealer.

• Why Used OEM? The voltage regulators in genuine Mitsubishi-made Subaru alternators are high quality and designed to work perfectly with the vehicle's sensitive electronics. Some budget aftermarket alternators can have inconsistent voltage output, leading to flickering lights or other electrical gremlins.
• Physical Inspection: Before buying, inspect the part. Look for a clean housing without significant corrosion or cracks. Check that the pulley spins freely and smoothly by hand, with no grinding or gritty feeling, which would indicate bad bearings.
• Mileage and Lifespan: Alternators can begin to fail after 80,000 miles, though many last much longer. When buying used, look for a part from a lower-mileage vehicle if possible.
• Verify Compatibility: This is the most important step. Double-check the part number against your original unit or use the seller's VIN-matching service. Pay close attention to the amperage and whether it's for a turbo or non-turbo model.

Alternator Replacement Cost Comparison

Prices can vary, but this gives a general idea of the cost for the part and labor.

Key Steps for DIY Replacement

Replacing the alternator on these Subarus is a very feasible DIY job for those with basic mechanical skills. The alternator is located on the top front of the engine, making it easy to access.

1. Disconnect the negative battery terminal.
2. Remove the plastic accessory belt cover (typically two 10mm bolts).
3. Loosen the accessory drive belt. On older models with manual tensioning, you'll loosen a 12mm lock bolt on the front of the alternator bracket, then turn the long 12mm tensioning bolt counter-clockwise to create slack. On newer models with an automatic tensioner, use a 14mm or 15mm wrench on the tensioner pulley bolt to rotate the entire assembly and slip the belt off.
4. Unplug the main electrical connector and unbolt the main power wire (usually a 12mm nut under a rubber cap).
5. Remove the main alternator mounting bolt and lift the alternator out.
6. Installation is the reverse of removal. Ensure the belt is routed correctly and tensioned properly.