Maintaining power to a car’s computer systems when disconnecting the battery is crucial to prevent data loss. Many vehicles achieve this through a constantly powered accessory outlet. However, this isn’t the case with Subarus, including the 99 Subaru Forester. This often leads to issues like an OBDII reader not working.
A common workaround involves using the OBDII connector itself. Pin 16 on the OBDII port maintains a constant connection to the car’s electrical system. Technicians often utilize special adapters to connect a 12V power source, like a jump starter, to this pin via a 7.5 amp fuse. This keeps the car’s modules powered, preserving learned data and settings like radio presets and the clock.
However, this method has limitations. The 7.5 amp fuse can easily blow if any significant electrical load is introduced. For instance, accidentally opening a door and activating the interior lights can exceed the fuse’s capacity. This results in a complete power loss to the modules, similar to disconnecting the battery. Consequently, the clock and radio presets reset, and the engine may idle roughly until the ECM relearns its parameters.
A blown fuse in this circuit directly impacts the functionality of an OBDII reader. Many code readers rely on the car’s power supply to operate. If the fuse is blown, the reader won’t power on, preventing it from communicating with the car’s computer and reading diagnostic trouble codes. This is a common reason for a 99 Subaru Forester Obdii Reader Not Working after battery replacement.
While a blown fuse related to the OBDII power supply doesn’t directly trigger a Check Engine Light (CEL), it hinders diagnosis. Therefore, if your OBDII reader isn’t working on your 99 Subaru Forester, check the relevant fuse in the fuse box first. Replacing this fuse might be the simple solution you need. If the fuse continues to blow, it indicates a short circuit that requires further investigation.
