Furnace Repair in Modesto, CA
Most Modesto furnaces sit idle from March to October and then get asked to run all night the first week the fog settles in. That is when the weak part shows itself, and it is why furnace repair searches here jump eightfold between August and November.
Why they fail on the first cold night
A gas furnace in the Valley does most of its year's work in about ten weeks. Through spring, summer and early fall it sits in the garage or the attic doing nothing, or running only its blower for the air conditioning. Dust settles on the flame sensor. The hot-surface igniter, a brittle ceramic element, has cooled and heated a few hundred times since its last season. Mice find the cabinet in the hall closet warm and quiet.
Then a November night drops into the high 30s, the thermostat calls for heat for six hours instead of twenty minutes, and anything marginal gives up. In the research behind this site, searches for furnace repair in Modesto ran 590 in October 2025, 720 in November and 480 in December, against 90 in August 2026. The pattern is the same every year, and it means the first cold week is also the week every heating company in Stanislaus County is busiest.
The practical answer is to turn the heat on for an hour in October, on a mild afternoon, while there is still time. If it is going to fail, that is the cheapest moment to find out.
Read the symptom before you call
Describing what the furnace does, in order, saves time on the phone and often narrows the repair before anyone arrives. These are the patterns technicians hear most, and what they usually point to.
The blower runs but the air is cold
On most furnaces the burners light first and the blower follows a minute later. If the blower runs and nothing warm ever comes out, the burners either never lit or lit and shut off. A dirty flame sensor is the most common reason: the furnace lights, cannot confirm the flame, and shuts the gas off within seconds as a safety measure. Cleaning or replacing the sensor is one of the cheaper repairs on this page.
Clicks, a hum, then nothing
The inducer motor starts (a hum or whir), the pressure switch closes (a click), and the igniter should glow. If it stops there and tries again a few minutes later, the usual suspects are a cracked igniter, a pressure switch that will not close, or a blocked vent or condensate line on a high-efficiency furnace. Many furnaces flash a fault code through a small window on the panel; reading out the number of blinks helps.
It runs a few minutes, shuts off, starts again
Short-cycling usually means the furnace is overheating and its high-limit switch is shutting the burners down. The cause is nearly always airflow: a clogged filter, closed registers, a crushed duct in the attic, or a blower that is failing. Left alone, repeated overheating is hard on the heat exchanger, which is the one part you do not want to replace.
Nothing happens at all
No hum, no click, no blower. Start with the thermostat (set to heat, set above room temperature, batteries alive), the furnace's power switch, the breaker, and the door on the furnace itself, which presses a safety switch that keeps everything off when the panel is loose. If all of that checks out, a transformer, a blown low-voltage fuse or the control board is likely.
A smell on the first run
A dusty, burning smell for the first twenty minutes of the season is normally dust burning off the heat exchanger and is not a fault. A smell that persists, smells electrical, or smells of gas is different. Gas means leave the house and call the utility from outside.
Where the furnace sits changes the job
In a Modesto house the furnace is in one of three places, and each brings its own repair problems.
In the garage
Common in the 1970s and 1980s tracts across north and east Modesto. Garage furnaces are the easiest to reach and the most exposed: they pull in garage dust, and anything stored against the cabinet blocks combustion air. They are also typically raised on a platform, because a gas appliance in a garage has to keep its burners away from vapours at floor level. If yours has been lowered or boxed in during a remodel, mention it.
In a hall closet
Usual in older and smaller homes. A closet furnace needs combustion air through louvered doors or vents, and a closet that has become a storage cupboard starves it. Closet units also tend to be the oldest still running, and finding a replacement that fits the same footprint can decide whether a repair is worth doing.
In the attic
Typical in many two-storey homes from the 1990s onward, in Village One, Sylvan and Salida especially. An attic furnace is harder and slower to reach, which shows up in labour time. High-efficiency models up there drain water through a condensate line, and a blocked line or a full overflow pan will shut the furnace off; in a bad case it stains the ceiling below first.
Wall and floor furnaces in the older neighbourhoods
Not every Modesto home has a forced-air furnace. In the College Area, around Graceada Park and in parts of La Loma, some houses still heat with a wall furnace or a floor furnace, the kind with a grate in the hallway floor. These are simple appliances, and simple repairs on them are usually possible. The problems are age and parts: many are decades old, the pilot assemblies and valves for some are no longer made, and a floor furnace whose grate has been used as a step or a dog bed for forty years needs a careful look at the burner and the vent. A good technician will tell you plainly when a wall furnace is past saving, and the replacement may be a new wall unit rather than ducts through the whole house.
Carbon monoxide and the heat exchanger
A gas furnace burns fuel inside a sealed metal heat exchanger, and the room air passes over the outside of it. A crack in that metal can let combustion gases, including carbon monoxide, into the air the blower sends around the house. That is why a technician who finds a cracked heat exchanger will shut the furnace down and may tag it as unsafe to operate. It is not an upsell; the furnace genuinely should not run.
California has required carbon monoxide alarms in homes with a gas appliance, a fireplace or an attached garage since 2011, and most Modesto homes qualify on all three counts. Test yours before the heating season. If it sounds while the furnace is running, get everyone outside, leave the door open behind you, and call 911 or the gas utility from outside. Do not go back in to switch the furnace off.
A crack is not always visible, and the honest version of this diagnosis comes with evidence: a camera image, a combustion analyser reading, or flame behaviour that changes when the blower starts. If you are told the heat exchanger is cracked and shown nothing, it is reasonable to ask to see what they saw, or to get a second opinion before paying for a new furnace.
What a repair visit looks like
Most local companies charge a diagnostic or service-call fee to come out, and many credit it against the repair if you go ahead. The technician checks the thermostat signal, watches a full start-up sequence, reads any fault code, and tests the parts that sequence points to. You should get the price for the repair before the work starts, not after. Common parts (igniters, flame sensors, capacitors, pressure switches) are usually on the van, so many repairs are finished on the first visit. Motors, control boards and gas valves sometimes have to be ordered, particularly for older or less common brands, and that can mean a cold night or two. Ask whether a space heater makes sense in the meantime, and keep it clear of curtains and bedding.
Repair or replace
Furnaces generally last somewhere around 15 to 25 years, and a lot of Modesto's are in that window: the tracts built through the 1980s and 1990s are on their second furnace or still running a tired first one. Age alone is not a reason to replace. A 22-year-old furnace with a new igniter can run several more winters.
What tips the decision is a big repair on an old unit. A common rule of thumb is that when a single repair approaches half the cost of a new furnace, and the furnace is past about 15 years, replacement starts to make more sense. Heat exchanger failures nearly always land there. So do repeated control board or blower failures on a furnace that has needed something every winter.
Replacement brings in things a repair does not: a city permit, California's energy code checks on the ducts, and possibly the question of switching to a heat pump. The repair-or-replace guide and the replacement page cover that side.
Who should be doing the work
In California, heating and air conditioning work is licensed by the Contractors State License Board under the C-20 classification (warm-air heating, ventilating and air-conditioning). Any contractor taking on a job worth $500 or more in labour and materials has to be licensed, and nearly every furnace repair beyond a sensor cleaning gets there.
Checking takes two minutes. The CSLB's Check A License page at cslb.ca.gov lets you search by licence number, business name or a person's name, and it shows whether the licence is active, which classifications it carries, and the bond and workers' compensation status. The company you are put through to should hold a C-20 and should give you the number without being asked twice.
If the house is cold now
Do the quick checks first: thermostat, power switch, breaker, filter, furnace door. Write down what the furnace does when it tries to start and how old it is (the date is often on the label inside the door). Then call. You will be put through to one local heating and air company that covers your part of Modesto, and they will tell you when they can get there.
No heat tonight? Call now and describe what the furnace is doing. One local heating and air company takes it from there.
| Repair | Typical sign | Generally quoted |
|---|---|---|
| Diagnostic / service call | Any visit | $80–$150, often credited to the repair |
| Flame sensor clean or replace | Lights, then shuts off within seconds | $100–$250 |
| Hot-surface igniter | Inducer runs, no glow, no flame | $150–$350 |
| Pressure switch | Click, then retries; fault code | $150–$400 |
| Draft inducer motor | Loud whine or no hum at start-up | $400–$900 |
| Control board | Dead or erratic; no fault code | $300–$800 |
| Blower motor | Burners light, no air moves; overheats | $450–$1,200 |
| Gas valve | Igniter glows, no gas | $350–$900 |
| Cracked heat exchanger | Found on inspection; CO concern | Usually means replacing the furnace |
Where we work in Modesto
- College AreaWest of downtownearly-1900s homes around the MJC East Campus, built long before central air; the ducts were added later, usually through tight attics or crawlspaces
- Graceada ParkCentralCraftsman-era homes around the park; air conditioning arrived decades after the houses did, so check whether the ducts and the furnace were ever sized for it
- La LomaEast, along Dry Creekolder homes under big trees along Dry Creek; the shade helps in July, and the leaves end up packed into outdoor coils by fall
- Village OneEast, toward Oakdale Roada planned area whose subdivisions went in from the late 1990s, so many original furnaces and condensers are now at replacement age
- Del RioNorth, toward the Stanislaus Riverlarger homes around the country club, many with two systems or a zoned one, which makes a single failure harder to pin down
- SylvanNortheastmostly 1980s to early-2000s tract homes; an original outdoor unit from that era runs on R-22, which is no longer made, so a leak usually ends in replacement
ZIP codes served95350 · 95351 · 95354 · 95355 · 95356 · 95357 · 95358
Common questions
How much is furnace repair in Modesto?
Most repairs local companies quote fall between about $150 and $600 after the diagnostic visit. Motors, gas valves and control boards cost more, often $500 to $1,500, and attic furnaces cost more to work on than garage ones. A cracked heat exchanger usually means replacing the furnace.
Why does my furnace blow cold air?
Usually because the burners did not stay lit. A dirty flame sensor is the most common cause; the furnace lights, cannot confirm the flame and shuts the gas off. Also check that the thermostat fan is set to auto rather than on, which runs the blower all the time whether or not there is heat.
Can I clean the flame sensor myself?
Some homeowners do, with the power and gas off, using fine abrasive pad. If you are not comfortable opening the burner compartment, leave it. A sensor that keeps getting dirty can also be a sign of a combustion problem that cleaning will not fix.
Is it safe to keep resetting the furnace?
Once or twice, yes. Repeatedly, no. A furnace that keeps locking out is telling you something, and forcing it through restarts can overheat the heat exchanger or flood a high-efficiency unit with condensate.
Should I get a tune-up before winter?
It is the cheapest way to avoid a November breakdown. A September or October visit cleans the flame sensor, checks the igniter and the venting, and gets the furnace started while there is still time to order a part.
My furnace is 20 years old. Is it worth repairing?
Often, if the repair is small. A new igniter or sensor on an old furnace is reasonable money. A heat exchanger, or a second major part in two winters, is where most people start pricing a replacement.
Do furnace repairs need a permit in Modesto?
Ordinary repairs generally do not. Replacing the furnace does, and so does changing gas piping or venting. Check with the City of Modesto building division if the work is more than a parts swap.
How do I check that a company is licensed?
Use the CSLB's Check A License search at cslb.ca.gov. Look for an active licence with the C-20 classification, and check the bond and workers' compensation entries while you are there.
Are you the company that fixes the furnace?
No. We are an advertising and referral service. Your call goes to an independent local heating and air company, which quotes and does the work. We hold no contractor licence and do not charge you anything.
Other heating and cooling work in Modesto
- AC Repair in Modesto, CA
- Heat Pump and HVAC Repair in Modesto
- Furnace and AC Replacement in Modesto
- Furnace and AC Tune-Ups in Modesto
- Air Filtration for Modesto Homes
Also serving
Furnace Repair is available beyond Modesto, across Salida , Riverbank , Ceres and Turlock .
Get help with furnace repair
Say what the system is doing and a local heating and air company will get back to you about a visit. Answered Mon–Sat, 8am–6pm · after hours we call back the same day Or call (209) 842-0950. If you have no heat or no cooling right now, calling is faster.