Original complaint
A second-floor office thermostat was blank and the system was not operating normally.
- Thermostat display was blank
- Heavy biological slime was found in the condensate drain path
- Drain pan itself was relatively clean
Measurements taken
- Supply air after service
- 55°F
Diagnostic process
- 1Checked the condensate circuit because a float/safety can interrupt control power
- 2Vacuumed the drain line and removed heavy slime
- 3Treated the pan/drain path to reduce immediate recurrence
- 4Restored operation and verified cool supply air
Finding / confirmed status
A condensate restriction activated the safety/control interruption, making the thermostat appear to be an electrical/thermostat failure.
Repair / action taken
Cleared and treated the condensate drain path and verified cooling operation.
Final operating readings
- Supply air
- 55°F
- Thermostat/system status
- Restored
Technician notes
A blank thermostat can be a symptom of a wet-switch/float safety opening the control circuit. Check condensate before replacing controls.
- Trace why 24 V control power is missing before condemning a thermostat
- Drain slime can create intermittent control complaints
- Verify positive drainage after cleaning
Safety cautions
- De-energize equipment before opening low-voltage/control compartments where line voltage is also present
- Use approved drain-treatment products for the equipment/materials
Safety note
HVAC work can involve high voltage, rotating equipment, pressurized refrigerants, and combustion systems. Use this site as a reference only. Follow the equipment manual, applicable codes, PPE, lockout/tagout, and current refrigerant rules. Field cases and troubleshooting paths are case-specific and are not OEM procedures or universal targets.