Thermostat blank or no 24 V: guided troubleshooting
Dead thermostat display, or nothing responds to a call. Answer each question from what you see on site.
Step 1
Does the thermostat run on batteries, and are they fresh?
Did this solve it?
See every step in this flow
- Does the thermostat run on batteries, and are they fresh?
- Battery powered, batteries are old or unknown: Replace the batteries first
- Fresh batteries, or it's powered by the C wire: Is there 24 V between R and C at the thermostat?
- Is there 24 V between R and C at the thermostat?
- Yes: Power is reaching the thermostat
- No: At the furnace or air handler, is there 24 V between R and C on the control board?
- There's no C wire: No common wire
- At the furnace or air handler, is there 24 V between R and C on the control board?
- Yes: Open thermostat cable or a safety in series
- No: Is there 24 V at the transformer secondary?
- Is there 24 V at the transformer secondary?
- Yes: Is the low-voltage fuse on the board blown?
- No: Is there line voltage at the transformer primary?
- Is the low-voltage fuse on the board blown?
- Yes: Blown low-voltage fuse
- No: Transformer is good, board has no 24 V
- Is there line voltage at the transformer primary? Check with the blower door switch held closed.
- Yes: Failed transformer
- No: No line voltage to the unit
Where each path ends
Replace the batteries first
A blank battery-powered thermostat is most often dead batteries. If the display comes back and the system still doesn't respond, start this flow again from the 24 V checks.
Power is reaching the thermostat
With 24 V at R and C and a blank display, check that the thermostat is seated on its sub-base and the wires are tight under the terminals. If so, the thermostat has failed.
No common wire
A thermostat that needs constant power won't run on R alone. Use a spare conductor as C, or the manufacturer's power adapter. On boilers with zone relays the common has to come from the right place.
Open thermostat cable or a safety in series
24 V at the board and none at the thermostat means a broken R or C conductor, a bad splice, or a float switch wired in the R circuit that has opened.
Blown low-voltage fuse
Find the short before replacing the fuse: a thermostat wire rubbed through at the outdoor unit, a shorted contactor coil, or wires touching at the sub-base.
Transformer is good, board has no 24 V
Look between the transformer and the board's R terminal: a float switch, a door switch on the low-voltage side on some units, a loose plug, or a failed board.
Failed transformer
Line voltage in and nothing out is an open transformer. Find out why it failed before installing the new one; a short on the 24 V side will take out the replacement too.
No line voltage to the unit
Check the breaker, the service switch, and the blower door switch.