- 951.513.8476
- CA License #1070401
Five things you can check yourself in about ten minutes, in the order a licensed technician would check them, cheapest and safest first. Then the honest line where it stops being a homeowner job. No heat on a cold night is unpleasant, but a fair share of these end without anyone paying for a part.
Nearly every no-heat guide online is written for a climate where the furnace runs for months on end. Ours doesn’t. In the Temecula valley the furnace sits almost completely idle from about March to November while the air conditioner takes every single cycle, and then, on the first properly cold evening, it gets asked for full output from cold.
That long idle stretch is the actual cause of most of what goes wrong here. It’s not really a winter problem. It’s a summer problem that only becomes visible in winter.
The flame sensor oxidises
A thin film builds on the sensor rod over months of sitting. It’s still physically fine, but it can no longer prove there’s a flame, so the furnace lights and then shuts itself straight back off.
The filter never got changed
A filter fitted in spring has spent the whole cooling season collecting dust. By October it can be blocked enough to trip a safety limit, which reads to you as a dead furnace.
Everything stiffens
Blower bearings, a damper linkage, a sticky relay. Parts that move every day tend to keep moving. Parts that sit still for eight months sometimes don’t start again politely.
So the sequence below isn’t the generic one you’ll find on a manufacturer’s site. It’s ordered for a furnace that has just woken up after a long hot season, which is the situation you’re actually in.
Every one of these is outside the sealed part of the furnace. None of them needs a tool more serious than a screwdriver, and none of them involves gas or high voltage.
| Check | What you’re looking for | Time | |
|---|---|---|---|
| 1 | Thermostat | Set to HEAT, not Cool and not Fan. Fresh batteries. Screen alive and the setpoint above room temperature. | Two minutes |
| 2 | Breaker | A gas furnace still needs power for its controls and blower. Push it firmly off, then on. | One minute |
| 3 | Service switch | Looks like a light switch, sits on or beside the furnace, gets knocked off by people hunting for the garage light. | One minute |
| 4 | Air filter | Hold it to a light. No light through means it’s blocked, and a blocked filter trips a safety limit. | Five minutes |
| 5 | Gas valve | Handle parallel to the pipe means open. Across the pipe means closed. Look, don’t force. | One minute |
It matters. The thermostat and the breaker come first because they’re free and they’re the ones most often at fault in a house where nobody has touched the heating since last winter. The filter comes before the gas valve because a blocked filter actively shuts the furnace down, while the gas valve is either open or it isn’t and you’ll know in ten seconds. Working cheapest-first is how a technician does it too, and it’s why a careful ten minutes is worth spending before you pick up the phone.
If the furnace lights and then quits after a minute or so, note that down before you call. That specific pattern is called short cycling and it points at a small number of causes, usually a dirty flame sensor or a safety limit tripping on overheating. Telling someone that on the phone genuinely shortens the visit.
Been through all five and the house is still cold? Call (951) 513-8476 Free written estimate, flat price agreed before any work starts, and we’re C-20 licensed for the gas side.
Past the five checks you’re into the parts of a furnace that can hurt you or the house. That isn’t a sales line, it’s the reason the licence exists. In California this work needs a C-20 licence, and ours is CA Lic. #1070401.
Cleaning a flame sensor sounds trivial and is described that way on plenty of sites. It sits inside the burner compartment, next to an open gas flame, and getting it wrong means the furnace either won’t prove the flame at all or proves one that shouldn’t be there.
A crack in a heat exchanger is the one genuine carbon monoxide risk in a gas furnace. It cannot be assessed from outside the cabinet and it isn’t something to take a guess at.
The handle on the shut-off is yours. Everything past it is not, including the valve body and the manifold.
Boards fail outright and, more annoyingly, intermittently. Proving which one is happening takes a meter and a wiring diagram, not a reset.
And the one absolute rule: if you smell gas, get everyone out of the house first, then call SoCalGas or 911 from outside. Don’t flip switches on the way out and don’t start with a contractor. That call comes afterwards.
Worth knowing before the conversation rather than after. Published 2026 national figures put most furnace repairs between $150 and $900, with the average around $350 to $400 (HomeGuide, 2026). The parts that fail after a long dormant season are mostly at the cheap end of that.
Flame sensor
Roughly $80 to $250. The single most common ignition failure, and the one dormancy causes most reliably.
Igniter
Roughly $100 to $250. Heats past a thousand degrees on every start and cools right down again, so it’s the part that wears out first.
Blower motor
Roughly $400 to $900, and up to $1,300 for a variable-speed one. The expensive end, and much less common.
Labour sits on top at $75 to $150 an hour, and most contractors carry a minimum service charge, typically $75 to $200 in the trade (HomeGuide, 2026). That’s why an $80 sensor is never an $80 visit, and it’s a fair question to ask about when you book rather than discover later.
If the furnace is old enough that you’re weighing a repair against replacement, there’s a rough test you can apply yourself: multiply its age in years by the repair quote, and if the answer clears about $5,000, replacing is usually the better buy. Age matters for running cost too, because an older furnace at a low efficiency rating burns more gas for the same warmth, and that difference lands on the utility bill every month you keep it. It’s also worth asking what rebate money is on the table before deciding.
Want the number before you commit to anything? Call (951) 513-8476 We quote flat, in writing, before the work starts, and we’ll tell you if the honest answer is that the system is not worth the repair.
Because the cause here is dormancy rather than wear, this is one of the more preventable faults in the trade. A check before the heating season starts catches the oxidised sensor and the blocked filter while it’s still daylight and nobody is cold.
Two specific habits do most of the work. Change the filter at the end of the cooling season rather than the start of it, which is when it’s actually dirty. And run the heat for twenty minutes on a mild evening in early autumn, deliberately, to find out whether it still works while it’s merely inconvenient rather than urgent. Our winter maintenance checklist has the full sequence, and how to replace an air filter covers the one job on this page that’s genuinely a five-minute task.
If it’s the air conditioning that won’t start rather than the heating, the same guide for an AC that won’t turn on walks the cooling side, which fails for different reasons. And our heating services covers the wider picture if you’ve decided the system needs more than a fix.
Work through it in this order: the thermostat, the breaker, the service switch on the furnace itself, the air filter, then the gas valve. Those five are the homeowner-checkable items and they account for a lot of no-heat calls. If all five are right and nothing happens, the usual next suspect is the ignition side, and a dirty flame sensor is the most common failure there. That one needs somebody licensed.
Because that’s the first time all year they’ve been asked to do anything. In this valley the furnace sits nearly idle for about eight months while the air conditioning takes every cycle. A flame sensor oxidises quietly over a long hot summer, bearings stiffen, and a filter that went in during spring is now thoroughly blocked. None of that shows up until something calls for heat, so the fault and the first cold evening arrive together.
Yes, and it’s one of the more common causes worth ruling out yourself. A filter too clogged to pass air makes the heat exchanger run hotter than it should, and a safety limit shuts the furnace down rather than let it cook. Trane’s own troubleshooting guidance puts the filter on the homeowner list for exactly this reason. Hold it up to a light, and if no light comes through, replace it before you call anyone.
Follow the pipe to the furnace and look at the handle on the shut-off valve. When the handle runs parallel to the pipe, the valve is open. When it sits across the pipe at a right angle, it’s closed. Don’t force a stiff valve and don’t go looking for anything beyond that handle. If the valve is open and other gas appliances in the house work, your supply isn’t the problem.
It depends entirely on which part it is. Published 2026 national figures put most furnace repairs between $150 and $900, with the average near $350 to $400 (HomeGuide, 2026). A flame sensor or an igniter sits at the cheap end, roughly $80 to $250. A blower motor is the expensive one at $400 to $900, reaching $1,300 for a variable-speed unit. Labour runs $75 to $150 an hour on top.
The moment the answer involves gas, high voltage or anything behind the sealed panel. Thermostat, breaker, service switch, filter and the gas valve handle are all fair game. Burners, the flame sensor, the heat exchanger, the control board and the gas valve internals are not, and in California that work needs a C-20 licence. If you smell gas, leave the house first and call SoCalGas, or 911, before you call a contractor.
Written by the team at SoCal AC Guy, a C-20 licensed heating and air conditioning contractor covering Temecula, Murrieta, Menifee, Lake Elsinore and the surrounding valley. The business is owned and run by Jorge, CA Lic. #1070401. The homeowner checks above follow published manufacturer troubleshooting guidance from Trane and Bryant, and the prices come from published 2026 national cost data. More about us, or get in touch.
