HVACopediaNTI · Boiler troubleshooting
Display shows System Disconnected! constantly
What is going on
Display is powered but cannot talk to the controller.
Likely causes
- Communication wire off
- Blown 24 VAC fuse C
- Failed 24 VAC transformer
- Control board dead
Check in this order
- Check the 4 by 1 Molex connector behind the top of the display is seated
- Check the wires on the back of the display: green, red and black to terminals 1, 2 and 3
- Check fuse C
- Look at the power LED on the control and measure 24 VAC at J8 pins 1 and 2
- If 24 VAC is present, make sure plug J8 is seated, cycle power, and replace the control if it still does not run
Applies to: Trinity Tft60, Tft85, Tft110, Tft154, Tft201, Tft285, Tft340, Tft399
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Source: NTI Trinity Tft399 Installation and Operation Manual (2018-09-07), Sec. 16.0 Troubleshooting. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.