Fix it first
HVAC troubleshooting guides
Before a repair is a repair, it is a symptom. Each guide below starts with the checks that cost nothing, tells you what each one rules out, and says plainly where to stop and call a licensed pro. Only then does it price the fix.
By the HeatAndAirCost editors · Last updated October 5, 2026 · How we research prices
Emergency first
Smell gas or rotten eggs? Leave on foot and call 911 or your gas utility from outside. CO alarm sounding? Get outside to fresh air and call 911. Smoke or flames? Get out, stay out, call 911. The full official instructions are on the gas, CO alarm and burning smell pages.
Start with the symptom
Pick what the system is doing. Every guide is ordered cheapest cause first.
Furnace not turning on
Thermostat, power, and safety-switch checks that cost nothing before you call a pro.
Diagnose it →Furnace blowing cold air
Why a running furnace blows cold, ranked from the free fixes to the real repairs.
Diagnose it →AC not cooling
From a dirty filter to a low refrigerant charge, cheapest cause first.
Diagnose it →How to reset a furnace
The reset button, the power switch, and the lockout sequence, step by step.
Diagnose it →Noises, smells, leaks and alarms
When the system is doing something odd rather than nothing at all, start here. Each guide collects what official agencies and manufacturers say the symptom can mean. The full index with summaries is on the HVAC symptoms hub.
Smells and alarms: safety first
AC not cooling right
Noises from the AC
Furnace, heat pump and thermostat
Comfort and cost
The four free checks
Whatever the symptom, these come first. Confirm the thermostat is set correctly and its batteries are fresh. Check the furnace power switch on the unit itself. Check the breaker. Pull the air filter and hold it up to a light. A clogged filter alone can stop a furnace on its limit switch or freeze an evaporator coil solid, and it is the single most common cause of a call that did not need to happen.
Then decide: repair or replace
Once you know what failed, the next question is whether this system is worth spending on.
The $5,000 rule
Multiply system age by the repair quote to see whether repairing or replacing is the better spend.
Read the guide →HVAC lifespan
How long furnaces, air conditioners, and heat pumps last before replacement is due.
Read the guide →Replace furnace and AC together
When pairing both replacements in one visit saves money, and when it does not.
Read the guide →How to choose an HVAC contractor
Licensing, written quotes, and the questions that separate a good bid from a bad one.
Read the guide →Price the part you were quoted
When a technician names the failed component, check the number against a researched range. The full index is on the part replacement cost hub.
Capacitor replacement cost
The cheapest common failure on both furnaces and air conditioners, and the one to rule out first.
Price this part →Furnace blower motor cost
PSC and ECM motor pricing, and when the blower wheel or a capacitor is the real fault.
Price this part →Furnace inducer motor cost
What the draft inducer costs to replace when the furnace locks out before ignition.
Price this part →Furnace pressure switch cost
A cheap safety switch that often gets replaced when a blocked vent is the actual problem.
Price this part →AC compressor replacement cost
The most expensive single AC repair, and the point where replacing the unit starts to win.
Price this part →Where to go next
For whole-job pricing, the cost guides hub has every researched 2026 range we publish, starting with furnace repair cost and AC repair cost. For what a repair tends to run in your metro, see the service areas hub.
Every range on these pages is compiled from published cost research and cited on the page it appears on. Read how we research prices or who runs HeatAndAirCost. We are not a contractor and the figures are researched ranges, not quotes.
People also ask
Troubleshooting FAQs
What should I check before calling an HVAC technician?
Four things, in this order: the thermostat is set to heat or cool and its batteries are good, the furnace power switch on the unit is on, the breaker has not tripped, and the air filter is clean. Each takes a minute and costs nothing, and any one of them can be the whole problem. A service call you avoid is $75 to $200 saved.
When is an HVAC problem a safety issue rather than a repair?
Stop and call a licensed pro immediately for a gas smell, a carbon monoxide alarm, scorching or soot around the furnace, or a cracked heat exchanger. Leave anything involving gas lines, refrigerant, or electrical repair to a technician. The free checks on these pages are deliberately limited to things that are safe to do yourself.
How many times can I reset a furnace?
Once. A furnace that locks out is protecting itself, and one reset tells you whether the lockout was a one-off. If it locks out again, something is actually wrong and repeated resets can make it worse. The reset guide walks through the safe sequence and where to stop.
Checks done, still broken?
Get free quotes from licensed pros
Know the fair 2026 range before you pick up the phone, then get connected with independent, licensed HVAC contractors. Free to you.