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Furnaces in McHenry County run from October through May. That's eight months of the year, more than half, spent cycling on and off to keep up. A furnace in a milder climate gets long stretches of rest. Yours doesn't get that. More run time means more cycles on the igniter, more hours on the blower motor, more chances for a part to wear past where it should. When something starts acting up here, it's rarely bad luck. It's usually a part that's been working since fall and is finally done.
NOAA nClimDiv, 1991–2020 normals
A cracked heat exchanger gets worse every hour the furnace keeps firing.
Shut it down and get it looked at before running it again.
If your furnace came with the house, check when that was. A repair is worth paying for on a furnace that still has years ahead of it. Replacement makes more sense when a system is old and failing in more than one way. For most homeowners here, a service call to diagnose the problem is the right next step before anyone talks about replacing anything.
A furnace that's run since October has had months to wear a part down before it finally quits. Get it diagnosed and fixed properly, so the repair holds for the rest of the season.
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Before anyone comes out, there are a few things worth checking yourself. Start at the thermostat. Make sure it's set to heat, not cool, and the set temperature is actually above room temperature. A dead or miscalibrated thermostat can look exactly like a dead furnace. Next, check the air filter. A filter clogged with dust chokes off airflow, and some furnaces will shut themselves down rather than run with no air moving across the heat exchanger. If yours pulls out gray and matted instead of white, swap it. Then find the furnace switch, it looks like a light switch, usually mounted on or near the unit, and make sure nobody bumped it off. Check the breaker too. A furnace that trips its breaker repeatedly shouldn't be reset again and again; that's a sign to stop and call someone. If you have a gas furnace, look for a reset button on the inducer motor or a flashing light on the control board. Count the flashes if you can, the pattern usually corresponds to a fault code, and whoever you call will want to know it. Don't try to relight a pilot or reset a gas valve yourself if you smell gas. Leave the house and call the gas company first. Last thing: listen to how it fails. A furnace that tries to start, clicks, and shuts off is different from one that never attempts anything at all, and telling a technician which one you're hearing saves time on the visit.
A furnace rarely quits all at once. An igniter weakens a little more each time it fires. A flame sensor collects a film that throws off its reading. A blower motor bearing wears down heating cycle after heating cycle. Each of these can still limp along for a while, until the day it can't.
Finding the part that's actually failing matters more than treating the symptom in front of you. A furnace that short cycles could be a dirty flame sensor or a cracked heat exchanger tripping a safety switch, and those two repairs aren't close to the same job. A proper diagnosis traces the symptom back to the part causing it, instead of guessing.
Estimates for a typical McHenry County home, compared with an older unit built to the previous federal minimum, from EIA household energy data, Illinois fuel prices and NOAA climate normals. Your home will differ.
Repair makes sense when one part has failed and the heat exchanger is sound.
Replacement makes sense when the heat exchanger is cracked or the furnace keeps needing parts.
A diagnostic visit starts with questions, not tools. What were you hearing before it quit, did it try to start, how long had it been acting up. Then the technician pulls the cover and checks the obvious things first: power to the unit, the flame sensor, the igniter, whether the inducer motor spins freely. If there's a control board, they'll read the fault code off it before touching anything, since that narrows down where to look. From there it's testing the part the symptoms point to, not swapping parts to see what helps. A multimeter across the igniter tells you if it's drawing the right current. A pressure switch gets checked for continuity. If the heat exchanger is in question, that gets inspected closely, because a crack there isn't something to guess about. The visit ends with the technician telling you what's actually wrong and what it takes to fix it, not just that something's wrong.
