Display shows 'Cool On' flashing and AC does not start

Safety first: No need to open HVAC panels for this condition.

Steps to troubleshoot:

• 1. Wait the delay period: Many Resideo thermostats enforce a 3–5 minute compressor protection delay. Wait at least 5 minutes to see if the AC starts.
• 2. Verify system switch: Ensure the thermostat is set to Cool and the setpoint is at least 3°F (2°C) below room temperature.
• 3. Check for power issues: If the display dims or resets, there may be low-voltage issues. Verify the furnace/air handler breaker is on.
• 4. Review installer setup: In advanced settings, confirm the system type (conventional vs heat pump) and compressor stages are configured correctly per the Resideo manual.
• 5. Test fan: Set the fan to On. If the fan runs but cooling never starts after the delay, there may be an equipment or wiring issue beyond the thermostat.

When to call a professional: If 'Cool On' continues flashing indefinitely and the outdoor unit never starts, contact an HVAC technician to check low-voltage control wiring and the outdoor unit.