Rick's Free Auto Repair Advice

P0300 Subaru Forester

Diagnose and fix P0300 Subaru Forester condition

Subaru has issued a service bulletin 07-112-16R to address a phantom P0300 Subaru Forester misfire code in the vehicles listed below. Do NOT replace any spark plugs, sensors, ignition coils or do ANY diagnostic work to correct this P0300 random misfire code until you verify the alternator issue identified in this service bulletin

Subaru vehicles affected by the P0300 code

2014 to 2016 Subaru Forester vehicle with the 2.5L engine

What causes the P0300 Subaru Forester condition?

A vehicle’s alternator must produce a steady charging voltage in order to not interfere with other vehicle electronics. Since an engine produces power in a pulse every time a cylinder fires, the torque created on the alternator belt is not steady. To remove some of the pulse vibration, auto makers rely on a dampening belt tensioner

Illustration showing the plastic bushing critical to proper belt tensioner operation

The tensioning arm rotates around the yellow plastic pivot arm bushing

and an over-riding clutch assembly mounted on the front of the alternator. The over-riding clutch prevents the alternator rotor from speeding up during a power pulse and then slowing down after the power pulse. In addition to the mechanical features, alternators also include electronics to reduce AC ripple and rapid voltage changes that might otherwise be introduced every time you turn an electrical accessory on or off.

The original Subaru alternators are producing voltage swings as much a ±3-amps within a 5-sec. period.  That too great a swing and can cause electrical problems that result in a false P0300 Subaru Forester random misfire code.

To understand how varying current can result in a P0300 misfire code, you have to understand first how the engine computer tracks and detects misfires. One common technique is to watch the rate of rotation on the crankshaft position sensor. The ECM knows when it commands a cylinder to fire and expects to see a power contribution and speed up of the crankshaft after each combustion cycle. Then it expects to see a slight slowdown after the event. If the ECM doesn’t see the expected speed up, it thinks the cylinder has misfired. Another method is to monitor the primary and secondary coil voltages. In the case of a faulty alternator, the current swings cause the ECM to misinterpret the crankshaft position sensor readings.

How to diagnose the P0300 Subaru Forester problem

First, Subaru states that you should address any other stored trouble codes before relying on the advice in this service bulletin. Next, Subaru states that you should the SSM4 factory tool to check alternator current and roughness values. Finally, Subaru has included illustrations showing the new and old alternator designs.

P0300 subaru forester

Fix the Subaru P0300 issue

The new Subaru alternator is part # 23700AB030 and the new alternator belt is 23769AA05A.




Posted on by Rick Muscoplat

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