Subaru factory diagnostics combine Subaru Select Monitor software with a vehicle communication interface. The software generation, interface and available service functions vary with the vehicle and market; software is not the same thing as the interface hardware.
SSMIII, SSM4, SDI and DST-i
In September 2015 Subaru of America advised retaining the earlier Subaru Diagnostic Interface (SDI) for 2003 and earlier vehicles and recommended the Denso DST-i interface for 2004 and newer vehicles. At that time it described DST-i with Select Monitor III (SSMIII) for 2004–2015 vehicles and Select Monitor 4 (SSM4) for 2016 and newer vehicles. This is a dated US service recommendation, not a universal statement covering later releases or every market. Subaru of America TechTIPS, September 2015.
Subaru's 2022 installation instructions show the DST-i connected to the vehicle and a network-enabled laptop running SSM4 for immobilizer-related registration. The US aftermarket distributor reports that SSMIII is included in SSM4 releases and that supported SSM4 software can switch to the earlier interface when appropriate. Interface compatibility and authorised functions should be checked for the exact job. Subaru installation instructions; Subaru aftermarket software distributor.
Newer applications
A 2026 Subaru of America service communication separately names Select Monitor 5 (SSM5), SSM5 Offline and GTS+, alongside SSM3–4. Its account of an outage states that the offline application did not provide reprogramming or immobilizer procedures. Do not assume every newer vehicle uses the same application or that offline and online capabilities match; consult current model-specific service information. Subaru of America communication, October 2026; Subaru aftermarket software and hardware listing.