When a furnace or air handler stops responding to the thermostat, the control board gets blamed first because it is the expensive part on the list. Before you order one, check the wire between the thermostat and the air handler. Thermostat cable runs through attics, crawl spaces, and wall cavities where staples pinch conductors, drywall screws find them, and rodents chew through jackets. A single broken conductor can look exactly like a dead board.
Safety First
Shut off power at the air handler's service switch and the breaker before you pull a cover or disconnect any wire. The low-voltage side runs at roughly 24 volts AC, but the transformer that makes it is fed by line voltage, and the cover screws sit close to live terminals. Set the meter to AC volts for the live checks and to continuity or ohms for the cold checks. Never measure resistance on a circuit that has power applied.
Step One: Read the Board With Power On
Restore power and go to the control board with the cover off. Find the terminals labeled R (or Rh and Rc on some systems), C, W, G, and Y. Measure AC volts between R and C. You should see something close to 24 volts. If the board reads correctly, the transformer and board are alive, and the fault is likely in the wiring between the board and the thermostat, or in the thermostat itself.
If R to C reads zero or nothing at all, stop. Check the breaker, the furnace switch, and the transformer's own fuse or breaker, if it has one. A dead transformer is a different job from a broken thermostat wire, and the cable test will not help you there.
Step Two: Check the Thermostat End
Power off again. Remove the thermostat from its backplate and look at the terminals. Power on, then measure AC volts between R and C at the backplate. If the board shows 24 volts and the backplate shows nothing, the break is in the cable between the two points. If both show 24 volts but the thermostat still does nothing, the thermostat is the suspect, and the batteries or internal fault come next.
Step Three: Ring Out Each Conductor
Power off at the switch and breaker. At the air handler end, leave the wires disconnected from the board. At the thermostat end, leave the wires disconnected from the backplate. Twist or clip the R and C conductors together at the thermostat end. At the air handler end, measure resistance between the matching conductors. A good conductor reads at or very near zero ohms. An open reading means a break somewhere in the run.
Repeat for each conductor, pairing each wire with its partner at the far end. Do not assume the color code matches the terminal labels. Older runs were often wired by people who used whatever wire was on the spool, and a multi-conductor cable may have a color that means one thing at one end and something else at the other. Ring out every pair before you trust any label.
Step Four: Fix the Break
For a short run with an accessible break, cut back to sound copper, make the splice in a junction box, and use a listed connector sized for the conductors. Leave slack so the splice is not under tension when the attic floor shifts. For a long run, or one with damage you cannot reach, pull a new cable. Thermostat cable is inexpensive compared with a board, and a fresh run removes the question of hidden damage.
After repairs, power the system and watch the call light and the board LEDs through one heating and one cooling cycle. If a conductor was shorted to another, a control fuse on the board may have blown. Check it, and replace it only with the rating printed on the board. Repeated fuse failure means a short is still present, and the board is not the cause.
Where the Trade Is Heading
An ACHR News feature on HVAC contractors reflecting on the change since the 2000s describes how the field has moved. The summary notes that "technology, customer expectations, and workplace culture have changed" over that period. For a homeowner, the practical result is that more of the diagnosis can happen before a technician arrives. A meter, a labeled wire run, and a written list of what you measured give a tech a faster start, and they cut the odds of paying for a board that was never broken.
If the thermostat cable exits through a roof penetration that also needs resealing, have that work done separately by a roofing contractor such as Keys Roofing, rather than leaving a rooftop seal to be patched during an HVAC visit. Keep the electrical repair and the roof repair on their own invoices so each trade owns its work.
Further reading: a roofing contractor such as Keys Roofing.