If your furnace won’t turn on when you raise the thermostat in heating mode, don’t panic — there are several common (and fixable) reasons why this might happen. Whether you’re dealing with a newer high-efficiency unit or an older gas furnace, this guide walks you through the 8 most likely causes, from power issues to failed parts.

We’ll also help you understand what you can troubleshoot yourself and when it’s time to call a pro.

1. No Power to the Furnace

Even if your thermostat is lit up, your furnace may have no power. Here’s what to check:

Breaker: Make sure the circuit breaker for the furnace is not tripped.
Power switch: There’s often a service switch near the furnace (like a light switch) — make sure it’s on.
Door switch: If the access panel is open or misaligned, the door switch can cut power to the unit.

Tip: Thermostats that run on batteries can still appear powered up even when the furnace has no power.

2. Blown Low Voltage Fuse or Bad Transformer

Your furnace needs 24V power to communicate with the thermostat and start up. If the system has a blown fuse or a failed transformer:

Check the fuse on the control board (usually 3 or 5 amp). Replace if blown.
Measure voltage at R and C terminals — you should see 24–29V AC.
Test transformer: With power on, confirm 24V is coming off the transformer’s low-voltage terminals.

If you’re getting 0 volts at the R terminal but have 120V at the board, suspect the fuse or transformer.

3. Safety Switch or Float Switch Is Open

Modern furnaces are wired with safety devices that break the circuit if something’s wrong:

Condensate float switch: Sits in the drain pan and opens the circuit if water backs up.
Rollout switch or high-limit switch: Opens if the furnace overheats.

If any of these are open, power to the thermostat can be interrupted.

4. Damaged or Shorted Thermostat Wire

The wire running from the furnace to your thermostat might be compromised:

Stapled too tight: Crushed wires = open circuit or short.
Rodents: Mice or squirrels can chew through thermostat wiring.
Loose connections: Check for bad splices or terminals, especially at the control board.

A short between the R and C wire will often blow the fuse on the board.

5. Bad Thermostat

If the thermostat isn’t calling for heat:

Make sure it’s set to heat and the temperature is at least 2°F above room temp.
If it runs on batteries, replace them.
If there’s no common wire (C), a dead battery can cause signal loss.
Corroded or dirty terminal connections inside the stat can prevent contact between R and W wires.

You can manually test by jumping R and W together at the furnace terminal — if the inducer motor starts, the stat is the problem.

6. Control Board Not Sending Power to Inducer Motor

If the inducer motor doesn’t kick on at the start of a heat cycle:

Confirm 120V from the board to the inducer motor terminals.
If there’s voltage at the board but not at the motor, you may have a failed relay or solder joint.

In most cases, the board will need to be replaced — diagnosing relays or soldering may only be a temporary fix.

7. Failed Capacitor (If Inducer Uses One)

Some inducer motors use a run capacitor (usually 3–5 microfarads):

If it’s weak, the motor may just hum but not start.
Use a multimeter to check its capacitance — replace if off by more than 5–10%.
Always match the microfarad rating exactly, and meet or exceed the voltage rating.

Capacitor issues can lead to motor overheating or long-term failure if ignored.

8. Bad Inducer Motor

If all other components are working properly but the inducer still doesn’t run:

Windings may be damaged (often due to a failed capacitor).
Thermal limit may be stuck open.
Rotor may be seized due to dry bearings.

Some older inducer motors have oil ports — adding a few drops of oil can extend their life. If the rotor is seized, you’ll need to replace the motor.

How to Safely Diagnose Furnace Problems

Always turn power off at the breaker before inspecting or removing parts.
Use a multimeter to confirm line (120V) and control (24V) voltages.
Watch for LED error codes on the control board — these can guide your troubleshooting.
Consult the furnace wiring diagram inside the panel.

When to Call an HVAC Technician

You can safely check breakers, switches, thermostat batteries, and filters — but when it comes to electrical diagnostics or motor replacement, it’s best to contact a licensed HVAC technician.

Especially in winter, delaying a repair can lead to:

Frozen pipes
Overstressed backup systems
Carbon monoxide issues (if flame rollout or venting is compromised)

Final Thoughts

If your furnace won’t turn on, it’s likely due to power supply issues, thermostat communication problems, or failed startup components like the inducer motor or control board. Knowing how to trace the startup sequence can help you understand where the failure is occurring — and whether it’s something you can fix or need help with.

Still having trouble? Contact our team for fast furnace diagnostics and repair in NJ.