The AAF‑McQuay MicroTech II 060006301 Control Unit is a microprocessor‑based HVAC controller used on McQuay vertical self‑contained and classroom ventilator units to manage temperature, pressure, and energy‑saving modes. Common failure points for this class of controllers include power supply degradation (brownouts or overvoltage), capacitor or diode failures in the input stage, and firmware corruption that can cause loss of communication with field devices. Initial troubleshooting should verify 24 V DC supply continuity and voltage levels, inspect all I/O terminals for shorts or open circuits, and review any stored error codes via the local user interface or diagnostic port. If power and connections are clean, perform a firmware integrity check and consider re‑flashing the controller’s microcode.[oss-gpt-20b]