The manual says: “HOT- — thermal threshold exceeded. Reduced performance mode.”
The "HOT-" designation often refers to the electrical status of the circuit. An "Open" circuit means the electrical path is broken, preventing the glow plugs from heating up ("HOT"), causing the system to flag a fault. In some scan tools, it may display as HOT- to indicate the Heater Output stage is missing or inactive. P2df000 Volvo HOT-