Thermostat calls for heat but boiler or furnace does not start; external call indicator on boiler is inactive

Safety first: Turn off power to both thermostat and boiler before working on control wiring.

Steps to troubleshoot:

• Confirm that the thermostat display shows a heating call (flame icon or similar).
• At the thermostat, verify that the correct relay contacts (usually COM and NO) are used for the boiler call.
• Inspect the cable from thermostat to boiler for damage, staples through the cable, or junction boxes with loose connections.
• At the boiler control board, check that the thermostat wires are firmly attached to the designated terminals (e.g., TA/TB, T/T, or room stat input).
• Use a multimeter to check continuity of each thermostat wire from end to end; repair or replace any open cable.
• Check the boiler’s low-voltage fuse and replace if blown, then investigate why it blew (shorts, miswiring).