When disconnecting a car battery, power is lost to various control modules, erasing learned data that optimizes vehicle performance. This also affects settings like radio presets and the clock. Some vehicles have always-on accessory power outlets, allowing a 12V supply to maintain power to the electrical system and prevent data loss when the battery is disconnected.
However, Subaru Foresters do not have constantly powered outlets. An alternative power source is needed when disconnecting the battery. One option is the OBD2 port, specifically pin 16, which maintains a constant connection to the system.
Many service centers utilize adapters to connect a 12V source, like a portable battery, to the OBD2 port via the 7.5 amp fuse. This method effectively powers the modules with their minimal power requirements, as long as no additional electrical loads are activated.
Opening a door, for instance, triggers interior lights, potentially exceeding the 7.5 amp fuse limit. If the current draw surpasses this limit with the car battery disconnected, the fuse will blow, resulting in data loss similar to a complete battery disconnect. This often manifests as reset clocks and radios, and occasionally rough idling until the ECM relearns its data.
A blown OBD2 fuse might explain a “Forester Obd2 Port No Power” issue. A shop may have attempted to maintain power through the OBD2 port during battery replacement but inadvertently overloaded the circuit. While a blown fuse won’t directly trigger a Check Engine Light (CEL), it will prevent OBD2 code readers reliant on the car’s power from functioning.
A simple test to determine if the fuse is blown is to check the clock and radio presets. If they have been reset, it’s highly likely the fuse has blown.
Replacing the 7.5 amp fuse associated with the OBD2 port is usually straightforward. Consult your Forester’s owner’s manual for the precise fuse box location and fuse number. Remember to disconnect the battery before replacing any fuses. Using a fuse with a higher amperage is not recommended, as it could damage other components.
If the issue persists after replacing the fuse, further diagnosis by a qualified technician may be necessary to rule out other potential problems with the vehicle’s electrical system. A multimeter can be used to verify power at the OBD2 port.
