The OBDII port on a 1999 Subaru Forester allows for connection to diagnostic tools. However, it can also be used to maintain power to the vehicle’s computer systems during a battery replacement, preventing the loss of learned data and settings. This is crucial because disconnecting the battery in a Subaru, unlike some other vehicles, results in the loss of stored information in various control modules, affecting optimized performance, radio presets, and the clock.
Subaru vehicles do not have constantly powered accessory outlets, making it impossible to maintain power through this conventional method. Therefore, an alternative power source is needed when disconnecting the battery. The OBDII connector, specifically pin 16, offers a constant power supply. This allows for a connection to a 12V source, such as a portable battery, via an adapter and a 7.5 Amp fuse.
This method effectively powers the car’s essential modules, requiring minimal current. However, caution is advised. Activating any significant electrical load, like interior lights triggered by opening a door, can exceed the 7.5 Amp fuse capacity, causing it to blow. This power interruption will result in the same data loss as a direct battery disconnection—resetting the clock, radio presets, and potentially causing rough idling until the Engine Control Module (ECM) relearns its parameters.
A blown fuse in this scenario is a common occurrence, often resulting from inadvertently triggering an electrical component while the main battery is disconnected. While a blown OBDII fuse won’t illuminate a Check Engine Light (CEL), it can prevent code readers that rely on the vehicle’s power from functioning. It’s possible a blown fuse explains why your settings were lost during a recent battery replacement. The technician may have attempted to maintain power through the OBDII port but accidentally overloaded the circuit.
