AC and Furnace Not Turning On: Troubleshooting Guide for Homeowners

The guide helps identify why the AC and furnace are not turning on, offering step-by-step troubleshooting, safety tips, likely causes, repair cost ranges, and when to call a professional. Readers will find actionable diagnostics for both central HVAC systems and common split systems to restore comfort quickly.

Issue	Common Causes	First Steps
Both AC And Furnace Won’t Start	Power loss, tripped breakers, thermostat issues	Check breakers, reset, inspect thermostat
AC Won’t Start, Furnace Works	Blown AC fuse, condenser issues, capacitor failure	Inspect outdoor unit, listen for attempts to start
Furnace Won’t Start, AC Works	Ignition failure, gas supply, flame sensor	Check gas valve, pilot/ignition status

How To Use This Troubleshooting Guide

The guide walks through easy visual checks, basic tests requiring no tools, and advanced diagnostics that may need a multimeter or HVAC tech. It emphasizes safety and shows how to narrow down causes quickly so homeowners can decide whether a DIY fix is feasible or a licensed professional is required.

Initial Safety Checks Before Troubleshooting

Before any inspection, ensure the power to the HVAC system is off at the main panel if accessing components. For furnaces, turn off the gas supply if any smell of gas is present and evacuate until the utility company confirms safety.

Check for tripped ground-fault devices, gas leaks, electrical burning smells, and visible damage. If any of these are present, stop and call a professional immediately.

Common Reasons Both AC And Furnace Are Not Turning On

When both heating and cooling systems are dead, the cause is often an electrical or control issue that affects both systems rather than a component specific to heating or cooling.

1. Tripped Circuit Breaker Or Blown Fuse

A tripped breaker or blown fuse that supplies the HVAC’s dedicated circuit will stop both systems. Breakers can trip due to surges, capacitor failures, or motor lockups.

Check the electrical panel for tripped breakers labeled for HVAC, furnace, or air conditioner. Reset a tripped breaker once; repeated trips indicate a deeper issue and need for a technician.

2. HVAC System Switch Or Disconnected Power

Most HVAC systems have a dedicated switch near the furnace or air handler and a disconnect at the outdoor condenser. These can be accidentally turned off during maintenance or by pets/children.

Verify all local switches and the outdoor disconnect box are in the ON position. Ensure no locked-out service switch is activated after recent work.

3. Thermostat Power Or Wiring Problems

If the thermostat loses power or has faulty wiring, neither the furnace nor the AC will receive a call to start. Battery-backed thermostats may display blank screens when batteries fail.

Check thermostat settings, replace batteries, and inspect visible wiring. For smart thermostats, reset to factory defaults if the interface is frozen, but record settings first.

4. Blown Low-Voltage Fuse (24V Fuse)

HVAC systems often use a 24-volt control circuit protected by a small inline fuse on the furnace or air handler control board. If this fuse blows, both systems lose control power.

Locate and inspect the 3- or 5-amp fuse on the control board. Replace with the exact type and rating only. If it blows again, do not continue replacing fuses—call a pro.

Why The AC Works But The Furnace Doesn’t

When the air conditioner runs but the furnace does not, the issue usually lies in fuel/ignition or furnace-specific controls rather than whole-house power.

1. Gas Supply Or Valve Issues

No gas or a closed gas valve will prevent a gas furnace from igniting. The pilot or electronic ignition needs gas to start the furnace burner.

Check if the household gas is on and ensure the furnace gas valve is open. If there is any doubt or smell of gas, leave the area and contact the gas company and an HVAC technician.

2. Ignition Control Or Hot Surface Igniter Failure

Modern furnaces use electronic igniters or hot surface igniters rather than pilot lights. These elements can fail with age or due to electrical problems.

Users may hear the furnace try to start (clicking) with no ignition. Replace igniters only with compatible parts and power off before replacing.

3. Faulty Flame Sensor Or Safety Lockout

A dirty or failing flame sensor may cause the furnace to shut down after ignition, then lock out the system until reset. Flame sensors are small rods near the burner that detect combustion.

Lightly clean flame sensors with fine-grit sandpaper as a temporary fix; persistent issues require a technician to test and replace the sensor.

Why The Furnace Works But The AC Doesn’t

If heating functions but cooling does not, problems usually relate to the outdoor condenser, refrigerant system, or AC-specific electrical components.

1. Condenser Unit Not Receiving Power

An outdoor unit may have a tripped breaker, blown fuse, or a disconnect switch turned off. Animals and weather can also damage disconnects.

Inspect the outdoor unit for power; listen for humming when the thermostat calls for cool. If the condenser is silent, check the disconnect and circuit breaker.

2. Failed Capacitor Or Contactor

Start/run capacitors and contactors are common failure points for outdoor units. Symptoms include humming without the fan or compressor starting, or visible pitting on the contactor.

Capacitor replacement requires safe discharge procedures and is recommended for licensed technicians due to high stored energy.

3. Refrigerant Leak Or Compressor Failure

Low refrigerant from a leak or a seized compressor will stop cooling. Refrigerant problems require EPA-certified technicians to repair and recharge systems.

Signs include warm air from vents, ice on refrigerant lines, or unusual noises from the outdoor unit.

Step-By-Step Homeowner Troubleshooting Checklist

1. Check Circuits: Verify breakers and fuses in the main panel and the outdoor disconnect.
2. Verify Thermostat: Replace thermostat batteries, ensure mode and temperature settings are correct.
3. Inspect 24V Fuse: Locate and check the control board fuse on the furnace/air handler.
4. Confirm Power Switches: Ensure the furnace switch and outdoor unit switch are ON.
5. Check Air Filter: A clogged filter can cause overheating and safety shutdowns; replace if dirty.
6. Listen For Operation: Note sounds when system attempts to start—clicks, hums, or silence indicate different faults.
7. Look For Error Codes: Modern furnaces/air handlers often blink LED codes on the control board—record the blink pattern for techs.
8. Reset Safeties: After ensuring safety, reset the thermostat and breakers once to see if system restarts.

When To Call A Professional HVAC Technician

Contact a licensed HVAC technician when the issue involves gas components, refrigerant systems, high-voltage electrical parts, repeated fuse or breaker trips, or if inspections reveal burning smells or visible damage.

Immediate professional help is necessary for gas leaks, electrical arcing, or if safety devices have been repeatedly tripped. Licensed techs have tools like multimeters, manometers, and refrigeration gauges required for safe diagnostics.

Estimated Repair Costs And Timeframes

Costs vary by region and the specific failure. These are ballpark ranges for the U.S. market and exclude emergency or after-hours rates.

Problem	Typical Cost Range	Typical Time
Reset Breaker/Fuse Or Thermostat Fix	$0–$150	30–90 Minutes
Replace 24V Fuse Or Minor Wiring Repair	$100–$300	1–2 Hours
Replace Capacitor Or Contactor	$150–$450	1–3 Hours
Igniter Or Flame Sensor Replacement	$150–$500	1–2 Hours
Compressor Replacement	$1,200–$3,500+	4–8+ Hours
Refrigerant Leak Diagnosis & Recharge	$200–$1,500+	2–6+ Hours

Preventive Maintenance To Avoid No-Start Situations

Regular preventive maintenance reduces the chance of both AC and furnace failures and extends equipment life. Seasonal tune-ups catch small issues before they cause system-wide shutdowns.

• Change air filters every 1–3 months depending on load and indoor air quality.
• Schedule annual furnace and AC inspections and cleanings with an HVAC contractor.
• Keep the outdoor condenser free of debris and vegetation; maintain 2–3 feet of clearance.
• Inspect electrical panels and connections during professional visits to catch corrosion and wear.
• Install a smart thermostat to receive alerts and improve diagnostics for remote troubleshooting.

Safety Reminders And Legal Considerations

Electrical and gas work often requires licensed professionals and permits. Homeowners should avoid work beyond basic inspections, filter changes, thermostat battery replacements, and reset of breakers.

Handling refrigerant requires EPA certification. Improper handling of gas or electrical systems risks fire, explosion, or serious injury, and may void warranties or violate local codes.

Key Diagnostic Tools And Their Uses

A few basic tools allow safer and faster diagnostics by homeowners and professionals.

• Multimeter: Tests voltage and continuity on thermostats, fuses, and starters.
• Thermostat Level And Batteries: Confirms proper settings and power for control signals.
• Flashlight And Mirror: Inspects burners, pilot assemblies, and control boards.
• Screwdriver Set: Access panels and disconnects safely.
• HVAC App Or Notebook: Record error codes, sounds, and sequence of failures for techs.

FAQ: Quick Answers To Common Questions

Why Do Both Systems Stop Working At The Same Time?

Most commonly a shared power or control issue such as a tripped breaker, a blown 24V fuse, or thermostat failure is to blame. These components control both heating and cooling operations.

Can A Dirty Filter Cause The System Not To Turn On?

A severely clogged filter can cause a furnace to overheat and trip safety switches, preventing operation. It is less likely to stop an AC from starting completely but can cause performance and safety shutdowns.

Is It Safe To Replace The Capacitor Or Igniter Myself?

Capacitors store dangerous charge and should be handled only by trained individuals. Igniter replacement may be simpler but still requires knowledge of system wiring and safe power isolation.

How Long Will It Take To Get Service?

Response times vary. Nonemergency service visits may be scheduled within 24–72 hours; emergency or after-hours calls can get faster but cost more.

Preparing For A Technician Visit

When professional help is needed, provide the technician with system make/model, age, recent symptoms, any error codes, and actions already taken. This expedites diagnostics and reduces onsite time.

Ensure access to the furnace/air handler and outdoor unit, and note any household disruptions such as pets or locked gates that could delay service.

Useful Links: Homeowners may consult manufacturer manuals for model-specific troubleshooting, and the U.S. Department of Energy for guidance on HVAC maintenance and energy-saving practices.