MOST (Media Oriented Systems Transport) is the high-speed multimedia network protocol used to connect infotainment modules on GM HMI-based vehicles. On a GM vehicle, the MOST bus connects the HMI, the radio (ACM), the amplifier (on Bose/premium audio), the media disc player, and other multimedia modules in a closed ring topology where each module both receives data from the previous module and forwards data to the next.
GM’s MOST implementation uses copper conductors (electrical signaling). The bus is designed to carry the high bandwidth needed for digital audio routing — particularly between the radio and the external Bose amplifier on premium audio configurations.
A break, short, or open anywhere in the MOST ring takes down the entire infotainment system. Because the ring is sequential, a single bad module or bad connector can make every other module on the bus appear to have failed. Common failure presentations:
- All infotainment dead, even though the HMI itself is healthy
- Missing audio with HMI screen still working (often a MOST issue, not an ACM issue)
- Intermittent infotainment that comes and goes with vibration
- “Audio system error” or similar messages
MOST faults are a frequent root cause of symptoms that look like a failed HMI but aren’t. WAMS requires a GM dealer diagnosis before shipping a replacement HMI specifically because a MOST bus open/short can present identically to module failure — and replacing the HMI will not fix a wiring problem.
On Global B vehicles, GM moved away from MOST in favor of Ethernet-based audio/video transport on the CSM. MOST bus is therefore an HMI-era concept.