It’s a common point of discussion among car enthusiasts and mechanics alike: Subaru and OBD2 compliance. While debates can sometimes arise, the reality is that all modern Subarus adhere to OBD2 standards, as mandated by law. However, the story doesn’t end there, especially for those diving into deeper diagnostics and ECU tuning. Let’s clarify the nuances of the Subaru Obd2 Protocol and explore why there’s more than meets the eye.
Subaru vehicles, particularly models like the Impreza, have incorporated an OBD2 diagnostic mode since the early 2000s, with the “bugeye” generation in 2001 marking a clear point of standardized access. In fact, some normally aspirated Subaru models in the US market had OBD2 capabilities even a couple of years prior. However, it’s crucial to understand that Subaru’s implementation is not simply a standard OBD2 system. It’s an OBD2 layer built on top of their pre-existing, proprietary system known as Subaru Select Monitor (SSM).
The Subaru Select Monitor (SSM) is a diagnostic protocol unique to Subaru, offering faster communication speeds and more comprehensive data access compared to generic OBD2. This is why tools like ROMRaider, ECUExplorer, EcuTeK/DeltaDash are favored by Subaru tuners and advanced users. These tools are designed to communicate using the SSM protocol, providing deeper insights and control. Consequently, standard ELM327-based OBD2 interfaces, which rely solely on OBD2 commands, cannot fully access the capabilities of the Subaru ECU in the same way. They are limited to the OBD2 command set, missing out on the richer SSM data.
It’s important to correct a common misconception: OBD2 isn’t just about Diagnostic Trouble Codes (DTCs). OBD2 is a complete standard that defines the entire communication methodology for vehicle diagnostics. This includes specifying the electrical interfaces, communication formats (like baud rates and initialization routines), and the language or commands used to request information from the Engine Control Unit (ECU).
Since 1999, Subaru has utilized the ISO9141 standard, part of the broader OBD2 specification, for the physical electrical connection between the ECU and external diagnostic tools. Initially, this ISO9141 connection was primarily used in conjunction with Subaru’s proprietary Select Monitor communication language. The full OBD2 specification, including the standardized diagnostic commands for emissions-related information, was implemented on turbocharged Subaru cars starting with the “bugeye” models in 2001. This implementation was not a replacement for SSM but rather an addition. For users who have the choice, especially for in-depth diagnostics and tuning, SSM remains the superior protocol due to its enhanced capabilities.
Confusion sometimes arises from older Subaru models. Vehicles like the 1999/2000 models and STi versions 5 and 6 may feature a physical connector that resembles an OBD2 port. However, attempting to use a generic OBD2 scan tool on these older models will likely prove unsuccessful. This is because, despite the presence of the physical port and the ISO9141 K-line connection, the ECU in these earlier turbocharged Subarus was not fully OBD2 compliant and will not respond to standard OBD2 initialization requests. Therefore, when discussing tools like ECUExplorer or ROMRaider, which are designed for Subaru vehicles, it’s essential to understand the context of SSM and the evolution of Subaru’s OBD2 implementation.
In conclusion, while all modern Subarus are OBD2 compliant, their diagnostic system is richer than standard OBD2 alone, thanks to the inclusion of the Subaru Select Monitor protocol. For basic diagnostics, generic OBD2 tools will suffice, but for comprehensive analysis, tuning, and deeper ECU interaction, tools leveraging SSM are essential for unlocking the full diagnostic potential of your Subaru.
