HVAC Troubleshooting Guide

Likely causes

Thermostat settings, dead batteries, a tripped breaker, an open furnace door switch, or a failed igniter or control board.

What you can try

1. Set the thermostat to HEAT and a few degrees above room temperature.
2. Replace the thermostat batteries.
3. Check that the furnace power switch (looks like a light switch nearby) is ON and the breaker hasn't tripped.
4. Make sure the furnace front panel is fully closed — a safety switch blocks startup if it's loose.

Likely causes

Fan set to ON instead of AUTO, a clogged filter, or a failed igniter, flame sensor, or overheating safety shutdown.

What you can try

1. Set the thermostat fan from ON to AUTO — on ON, the blower runs even between heating cycles and feels cold.
2. Replace a dirty air filter.
3. Give the burners a minute to ignite after the call for heat.

Likely causes

Overheating from restricted airflow (the number one cause), a thermostat in a bad spot, or a faulty flame sensor or limit switch.

What you can try

1. Replace the air filter — a clogged filter overheats the furnace and forces it to shut down early.
2. Open all supply vents and clear any blocked returns.
3. Make sure the thermostat isn't near a heat vent, lamp, or sunny window.

Likely causes

Older units: a dirty pilot orifice or failing thermocouple. Modern units: a cracked hot-surface igniter or a dirty flame sensor.

What you can try

1. On a standing-pilot furnace, follow the relight instructions printed on the unit exactly.
2. Keep the area around the pilot/igniter free of dust and drafts.

Likely causes

Banging on startup can be delayed ignition or ducts expanding; squealing usually means a worn blower belt or motor bearings; rattling is often loose panels or hardware.

What you can try

1. Tighten the furnace access panels and any loose screws.
2. Replace the filter — restricted airflow can cause whistling and strain.

Likely causes

Clogged filter, closed or blocked vents, leaky ducts losing heat in the attic or crawlspace, or an aging/undersized furnace.

What you can try

1. Replace the filter and open every supply register.
2. Feel along accessible ductwork for obvious leaks or disconnected sections.
3. Lower thermostat setbacks gradually rather than expecting instant recovery.

Likely causes

Fan set to ON, a stuck fan limit switch, or a wiring/relay fault on the control board.

What you can try

1. Switch the thermostat fan setting from ON to AUTO.
2. Cycle the system off and back on at the thermostat.

What it means

A healthy gas flame burns crisp blue. A yellow, orange, or flickering flame signals incomplete combustion, which can produce carbon monoxide.

What it means

A dusty burning smell the first time you run heat each season is normal and clears within an hour. A persistent electrical, smoky, or hot-plastic smell is not.

What you can try

1. For a brief dusty smell, replace the filter and let it clear.
2. For any electrical or burning-plastic smell, shut the system off immediately.

Likely causes

A clogged filter or failing blower motor drawing extra current, a short in the wiring, or an overloaded circuit.

What you can try

1. Replace the filter.
2. Reset the breaker one time.

What it means

The heat exchanger keeps combustion gases separate from the air you breathe. A crack can let carbon monoxide into your home.

Warning signs

• Soot around the furnace
• A strong chemical or formaldehyde-like smell
• CO detectors going off
• Headaches or nausea that improve when you leave the house

What it means

Most furnaces have a small LED on the control board that blinks a pattern when something's wrong. The blink pattern maps to a specific fault.

What you can try

1. Count the blinks (for example, three short flashes).
2. Open the lower access panel and read the code legend printed inside.
3. Common codes cover ignition lockout, pressure-switch faults, and limit-switch trips.

What it means

The flame sensor confirms the burner actually lit. Over time it builds a film that stops it from reading the flame, so the furnace lights then shuts off seconds later.

What you can try

1. Shut off power and gas to the furnace.
2. Remove the single screw holding the sensor (a thin metal rod in the flame path).
3. Gently buff it with fine steel wool or a non-abrasive pad, then reinstall.

Likely causes

Thermostat or fan setting, a dirty filter, a blocked or dirty outdoor unit, low refrigerant, or a frozen coil.

What you can try

1. Set the thermostat to COOL and the fan to AUTO.
2. Replace the filter.
3. Clear leaves and debris from the outdoor condenser and rinse the fins gently.

Likely causes

Dead thermostat batteries, a tripped breaker, the outdoor disconnect switch left off, a blown fuse, or a full condensate pan tripping the float switch.

What you can try

1. Replace thermostat batteries and set it to COOL.
2. Check the breaker and the outdoor disconnect box near the condenser.
3. Empty the condensate pan if it's full (a safety float can shut the system down).

Likely causes

Restricted airflow (dirty filter or coil) or low refrigerant. Ice blocks the system and can damage the compressor if you keep running it.

What you can try

1. Turn the AC OFF and set the fan to ON to thaw the coil fully (can take a few hours).
2. Replace the filter and make sure vents are open.
3. Restart in COOL once all ice is gone.

Likely causes

A clogged condensate drain line is by far the most common. A cracked drain pan, frozen coil, or failed condensate pump can also leak.

What you can try

1. Turn the system off to stop the leak.
2. Find the condensate drain line (usually a PVC pipe) and clear the clog with a wet/dry vacuum at the outdoor end.
3. Pour a cup of distilled vinegar in the access tee to discourage algae regrowth.

Likely causes

A failed run capacitor is the classic cause — the unit hums but the fan won't spin. A bad contactor, motor, or tripped disconnect can also be responsible.

What you can try

1. Turn the unit off so the compressor doesn't overheat.
2. Confirm the outdoor disconnect and breaker are on.

Likely causes

Dirty filter, frozen coil, low refrigerant, an oversized system, or a poorly located thermostat.

What you can try

1. Replace the filter and clear the outdoor unit.
2. Make sure the thermostat isn't in direct sun or near a vent.

What each sound usually means

• Buzzing: electrical issue, loose parts, or a failing capacitor.
• Rapid clicking: a failing relay or control.
• Hissing: a possible refrigerant leak or high pressure.

What you can try

1. Tighten any obviously loose access panels.
2. Turn it off if you hear hissing or persistent buzzing.

Likely causes

Clogged filter, a dirty blower wheel or coil, leaky or crushed ducts, or a failing blower motor.

What you can try

1. Replace the filter and open all registers.
2. Check accessible ducts for disconnections or kinks.

The short answer

No. Refrigerant handling requires EPA Section 608 certification and is regulated by law. Just as important: a low charge means there's a leak, and topping it off without finding the leak only masks the problem temporarily.

What it does

The capacitor gives the fan and compressor motors the surge they need to start and keep running.

Signs it failed

• The unit hums but won't start
• The fan needs a push to begin spinning
• A swollen or leaking capacitor top
• AC works intermittently

Yes — and you can do it safely

1. Shut off power at the outdoor disconnect.
2. Remove leaves, grass clippings, and debris from around and inside the unit.
3. Gently rinse the fins with a garden hose from the inside out — never a pressure washer.
4. Keep at least two feet of clearance on all sides and trim back plants.

Likely causes

A dirty condenser making the compressor work harder, a failing capacitor or motor, a refrigerant problem, or an electrical short.

What you can try

1. Clean the outdoor unit and replace the filter.
2. Reset the breaker one time only.

Likely causes

Undersized equipment, a dirty condenser or filter, low refrigerant, leaky ducts, or weak insulation letting heat in faster than the AC removes it.

What you can try

1. Replace the filter and clean the outdoor condenser.
2. Close blinds on sun-facing windows and limit heat-generating appliances midday.

Likely causes

Wrong thermostat mode, a dirty filter, snow or ice blocking the outdoor unit, a stuck reversing valve, or low refrigerant.

What you can try

1. Set the thermostat to HEAT and confirm it's calling for heat.
2. Replace the filter.
3. Clear any snow, ice, or leaves from around the outdoor unit.

Likely causes

Thermostat setting, a dirty filter, a blocked outdoor unit, a stuck reversing valve, or low refrigerant.

What you can try

1. Set the thermostat to COOL.
2. Replace the filter and clear the outdoor unit.

What's normal vs. not

A light frost that clears every hour or so during the defrost cycle is normal. Heavy ice that covers the whole coil for hours is not.

What you can try

1. Make sure nothing blocks drainage or airflow around the unit.
2. If heavily iced, run the system in defrost or turn it off to let it thaw.

What's normal vs. not

Heat pumps run longer than furnaces by design, and near-constant running in very cold weather is expected. Constant running in mild weather is not.

What you can try

1. Replace the filter and check that vents are open.
2. Look for obvious duct leaks losing conditioned air.

What it means

Auxiliary or emergency electric heat strips are far more expensive to run. They should only kick in during very cold weather, defrost cycles, or large thermostat jumps.

What you can try

1. Avoid big temperature setbacks that force a large recovery (which triggers aux heat).
2. Make sure you're not accidentally set to Emergency Heat mode.

What it means

A solid block of ice means the defrost system isn't working or water is pooling and refreezing around the unit.

What you can try

1. Switch to Emergency Heat or off to let the unit thaw completely.
2. Clear gutters or downspouts dripping onto the unit, and improve drainage at the base.

What you can try

1. Replace the batteries (if it takes them).
2. Check the breaker and the furnace door safety switch.
3. For smart thermostats, a missing or loose C-wire can cut continuous power.
4. A blown low-voltage fuse on the furnace control board can also cause this.

Likely causes

The thermostat is near a vent, window, lamp, or sunny wall; it's dusty inside; or the sensor is drifting.

What you can try

1. Gently clean dust from inside the cover.
2. Confirm it's on an interior wall away from heat and drafts.
3. Use the calibration or offset setting if your model has one.

What it is

The C-wire (common wire) provides constant low-voltage power that smart thermostats need to run their screen and Wi-Fi.

Your options

1. Use an existing unused wire in the bundle as a C-wire.
2. Install an add-a-wire adapter at the furnace.
3. Use the plug-in transformer some thermostats include.
4. Have a tech run a proper C-wire from the control board.

Likely causes

A lock or hold mode is active, a schedule is overriding you, or a temperature range limit is set.

What you can try

1. Look for a lock icon and disable keypad lock.
2. Cancel any "hold" and review the programmed schedule.
3. If stuck, a factory reset clears most lockouts.

A simple, efficient setup

1. Set comfortable temperatures for when you're home and awake.
2. Set the heat lower (and cooling higher) for sleep and while away — about 7-10 degrees of setback.
3. Schedule recovery to begin shortly before you wake or return.

This saves energy without forcing the system into a hard, expensive recovery.

Likely causes

Closed or blocked vents, a dirty filter, leaky ducts, weak insulation, or dampers out of balance.

What you can try

1. Open all registers at least partway and clear furniture from vents.
2. Replace the filter.
3. Adjust duct damper levers to send more air to problem rooms.

Likely causes

A clogged filter, a dirty blower wheel, undersized or leaky ductwork, or a failing blower motor.

What you can try

1. Install a fresh filter and open all registers fully.
2. Inspect visible ducts for leaks or disconnected runs.

Likely causes

A cheap or overdue filter, leaky return ducts pulling in attic or crawlspace dust, or dirty ductwork.

What you can try

1. Upgrade to a better-rated filter — but not so restrictive it chokes airflow.
2. Seal accessible return-air leaks.
3. Consider professional duct cleaning if buildup is heavy.

Likely causes

Air being forced through a restriction: a clogged filter, too many closed vents, or undersized return ducts.

What you can try

1. Replace the filter.
2. Open more vents to relieve pressure.

What you can try

1. Partially close registers in rooms that get too much air, sending more to the rooms that need it.
2. Use duct damper levers (often near the furnace) for bigger adjustments.
3. Keep every register open at least partway to avoid pressure problems.

For two-story homes, send more cooling upstairs in summer and more heat downstairs in winter.

The simple rule

• Standard 1-inch filter: every 1-3 months.
• With pets or allergies: monthly.
• Thick 4-5 inch media filter: every 6-12 months.

A clean filter is the single most important thing you can do to protect the system and keep bills down.

Homeowner checklist

• Replace filters on schedule.
• Keep the outdoor unit clear of debris and plants.
• Vacuum vents and returns.
• Test the thermostat and replace CO and smoke detector batteries.

Professional tune-up

Once a year is ideal — heating in the fall, cooling in the spring — to catch small issues before they become breakdowns.

Likely causes

A dirty filter, leaky ducts, low refrigerant, aging equipment, poor insulation, or a thermostat schedule that overworks the system.

What you can try

1. Replace the filter and clean the outdoor unit.
2. Set sensible thermostat setbacks.
3. Seal obvious duct and window leaks.

Aim for 30-50% indoor humidity

• Too humid (summer): a properly sized AC dehumidifies as it cools; an oversized one cools too fast to remove moisture. A whole-home or portable dehumidifier helps.
• Too dry (winter): a whole-home humidifier on the furnace, or standalone units in key rooms.

Quick guide

• Musty: mold in the coil, drain pan, or ducts.
• Dusty/burning (brief, first heat of season): usually normal.
• Burning plastic or electrical: shut it off and call a pro.
• Rotten egg / sulfur: possible gas leak.

Lean toward replacement when

• The system is over 12-15 years old.
• A repair costs more than about half the price of a new unit.
• It uses old R-22 refrigerant (expensive and phased out).
• Breakdowns are becoming frequent.

Newer high-efficiency systems often pay back part of their cost through lower energy bills.

What you can try

1. Keep snow and ice cleared from around a heat pump or condenser.
2. Never wrap or cover a unit that's still running.
3. Insulate exposed condensate and water lines.
4. Make sure nothing blocks airflow to the unit.

Steps

1. Turn the thermostat OFF.
2. Restore power at the breaker and the system's power switch.
3. Wait a few minutes.
4. Set the thermostat back to your mode and give it several minutes to restart.

Many systems include a built-in delay to protect the compressor — a pause before restart is normal.