Same-day shipping on orders before 3PM CST · 10,893 OEM Parts In Stock · Carrier · Bryant · ICP · Heil · Tempstar
Track Order | 1-800-NHP-HVAC | Trade Accounts

Menu

Home Shop All Account Cart

HVAC Diagnostic Guide HIGH URGENCY

Thermostat Not Cooling or Heating

Thermostat displays normally with correct mode set, but neither cooling nor heating activates.

Quick Check Before You Call a Pro

  1. Verify thermostat mode (HEAT/COOL/AUTO) AND setpoint relative to room temp
  2. Try setting setpoint 5°F more aggressive than current to force a call
  3. Check thermostat batteries (low battery prevents reliable signals)
  4. Inspect indoor unit's drain pan float switch — most common cause of total no-response

01 · Most Likely Causes (Ranked)

30%

Tripped Float Switch (Clogged Drain)

What's happening

Float switch in AC drain pan tripped, cutting 24V power to the entire low-voltage circuit including thermostat operation. Even though thermostat display still works (it has internal power), heating and cooling won't activate.

How to check

Locate indoor air handler. Find drain pan and float switch (small plastic device with wires). If float is UP, it's tripped. Standing water in pan = clogged drain.

How to fix

Clear drain line: wet/dry vac at outdoor end or vinegar in indoor port. Drain the pan. Reset float by lowering manually.

20%

Failed 24V Transformer

What's happening

Without 24V from the transformer, thermostat can call all day and nothing will respond. Thermostat may show low-battery warning or display dimly. Some units lose display entirely.

How to check

Multimeter at thermostat terminals R and C. Should show 24-28V AC. Zero or very low = dead transformer.

How to fix

Replace transformer. Match VA rating (40VA typical for residential). $25-50 part. Located inside furnace/air handler.

Shop Transformers
18%

Blown Control Board Fuse

What's happening

Small automotive-style fuse on furnace control board protects the 24V circuit. Blown fuse = no power to thermostat circuit = no response.

How to check

Power off at breaker. Open control panel. Look for small blade fuse (3A or 5A). Pull and inspect — burned filament = blown.

How to fix

Replace with same amperage. NEVER substitute higher amperage. If it blows again immediately, find the wiring short.

12%

Wrong Mode or AUTO Confusion

What's happening

Thermostat in OFF mode, or AUTO with current room temp between cooling and heating setpoints (deadband). System legitimately has no demand. Looks like failure but is normal.

How to check

Check mode display. OFF = no operation. AUTO with current room temp at 72°F, cooling setpoint 75°F, heating setpoint 68°F = no demand (within deadband).

How to fix

Switch to specific mode (HEAT or COOL) and set setpoint 5°F more aggressive to force a call. If system responds, original symptom was normal AUTO behavior.

Shop Thermostats
10%

Loose Thermostat Wires

What's happening

Wires at the thermostat or at the furnace control board have loosened from vibration. R wire loose = no 24V. Y wire loose = no cooling. W wire loose = no heating.

How to check

Pull thermostat off the base plate. Wiggle each wire — any loose connection? Check the same connections at the furnace control board.

How to fix

Reseat all wires firmly. Strip back 1/4 inch of wire if oxidized. Tighten terminal screws fully.

8%

Failed Thermostat

What's happening

Thermostat appears to function (display, buttons) but internal relays don't actually close to send calls. Common on aging or storm-damaged units.

How to check

At the furnace control board, jumper R to Y (for cooling test) or R to W (for heating test). If system responds when jumpered, thermostat has failed.

How to fix

Replace thermostat. Match wiring requirements (number of conductors, C-wire support, voltage).

Shop Thermostats
2%

Compressor/Heating Lockout

What's happening

Some thermostats have minimum cycle times or compressor protection delays (typically 5 minutes). If you just turned off heating or cooling, the lockout may be active.

How to check

Did you just power-cycle the thermostat or change modes? Wait 5-10 minutes and see if operation resumes.

How to fix

Wait for delay to clear. If you need to override, most thermostats have a temporary override option in the menu.

Need the Replacement Part?

OEM-quality HVAC parts shipped same day on orders before 3 PM CST. Contractor pricing available.

TransformersThermostats

02 · Frequently Asked Questions

Why won't my thermostat make the AC or furnace turn on?
Most common causes: tripped float switch in AC drain pan (cuts power to entire system), blown fuse on furnace control board, or dead 24V transformer. Check each in order — float switch first.
How do I test if my thermostat is the problem?
At the furnace control board (NOT the thermostat), use a piece of wire to jumper R to Y for cooling, or R to W for heating. If system responds when jumpered, thermostat is the problem. If not, problem is downstream.
What is AUTO mode on a thermostat?
Auto mode lets the thermostat automatically switch between heating and cooling based on indoor temperature and your setpoints. Requires a deadband (gap) between cooling and heating setpoints. Within the deadband, no operation occurs.
Should I have a C wire on my thermostat?
Smart thermostats almost always need a C (common) wire for continuous power. Older 2-wire thermostats steal power from R during inactive periods. C wire prevents this and improves reliability.

03 ·Related Symptoms

Thermostat Blank or Not WorkingAC Not Responding to ThermostatThermostat Reading Wrong Temperature
⚠ Safety notice: This guide is for informational purposes only. HVAC systems involve high-voltage electricity, refrigerants under pressure, and natural gas — all of which can cause serious injury, death, or property damage. Refrigerant handling requires EPA Section 608 certification by federal law. When in doubt, contact a licensed HVAC professional.
Home Shop Search Account Cart