Experiencing a “no communication” error when trying to connect your scan tool to your 2016 Subaru Legacy can be frustrating. Often, this issue surfaces unexpectedly, and one potential, yet commonly overlooked cause, is related to a recent battery replacement. Let’s delve into why changing your car battery might lead to this communication problem and how to troubleshoot it.
One of the fundamental aspects of modern vehicles like the 2016 Subaru Legacy is the presence of numerous electronic control modules. These modules are essentially small computers that manage various functions of your car, from engine performance to the radio settings. Many of these modules store learned data which helps optimize your car’s operation over time. This includes things like idle settings and fuel trim. Furthermore, features like your radio presets and clock also rely on a constant power supply to retain their settings. When the battery is disconnected, even for a short battery replacement, all of these volatile memory settings are lost.
To prevent this data loss during battery changes, some vehicles offer a convenient workaround: constantly powered accessory outlets. The idea is that you can connect a 12V power source to this outlet while the battery is disconnected, maintaining power to the system and preserving those crucial settings. However, Subaru vehicles, including the 2016 Legacy, are known not to have constantly powered accessory outlets. This means relying on the accessory outlet method for memory preservation during battery changes won’t work.
So, how can you maintain power during a battery replacement on a 2016 Subaru Legacy? A common alternative is to use the OBD-II (On-Board Diagnostics II) connector. Pin 16 of this connector is designed to be always powered, providing a potential point to introduce an external 12V source.
Many automotive service centers utilize specialized adapters that connect a 12V power supply, such as a jump starter, to the OBD-II port, specifically pin 16. This connection is usually fused at 7.5 Amps for safety. This method can successfully maintain power to the car’s computer systems while the battery is disconnected, as long as you avoid turning on any power-drawing accessories. The minimal current draw of the modules and memory is well within the 7.5 Amp fuse rating.
However, a common mistake can lead to problems. If, during the battery replacement process with OBD-II power maintenance, a door is opened, or interior lights are switched on, the increased electrical load can easily exceed the 7.5 Amp fuse’s capacity. When this happens, the fuse blows, effectively cutting off the temporary power supply.
The consequence of this blown fuse is twofold. First, just like disconnecting the battery without any power maintenance, you’ll lose your radio presets, clock settings, and potentially experience a slightly rough idle until the engine control module (ECM) relearns its optimal parameters. Second, and more relevant to the “no communication with scan tool” issue, if your scan tool relies on power from the car’s OBD-II port to operate, a blown fuse on pin 16 will prevent it from powering up and communicating with the vehicle’s systems.
Therefore, if you are encountering a “no communication” error with your scan tool on your 2016 Subaru Legacy after a battery replacement, checking the 7.5 Amp fuse associated with the OBD-II port (often related to the backup power or memory fuse) should be one of your first steps. It’s a simple issue to resolve, but easily missed and directly connected to the symptom of a scan tool failing to connect. While this fuse issue itself won’t trigger a check engine light, it will definitely prevent your scan tool from accessing any diagnostic trouble codes.
