October 5, 2026
Your AC stopped working and you're not sure if it's a real emergency. Here are five things to check before you pick up the phone.
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It’s a Tuesday night in late September. The temperature in San Jose dropped into the low 50s after a warm week, you turned on the heat for the first time since spring, and nothing happened. Or maybe it’s July, the valley is pushing 98°F, and your AC just quit. Either way, the instinct is to call someone immediately — and that instinct isn’t wrong. But before you do, there are five things worth checking first. Some of them take two minutes. Any one of them could be the reason your system stopped, and catching it yourself means no service call, no wait, and no fee.
The fear most homeowners have before calling isn’t really about the cost — it’s about calling a technician and being told the problem was a tripped breaker. That’s a real and reasonable concern, and it’s exactly why running through a short checklist before you dial makes sense.
None of these checks require tools or technical knowledge. They’re the same things our technicians will verify in the first few minutes on-site anyway. If you catch the problem yourself, great. If you don’t, you’ll have already ruled out the simple stuff, which actually helps us diagnose the real issue faster.
Start here, every time. The thermostat is the single most common source of a “my system stopped working” call that turns out to be nothing. It sounds obvious, but in the middle of a heat wave or on the first cold night of fall, it’s easy to overlook.
First, check the mode. If your thermostat is set to “cool” and you’re trying to run heat, the system won’t respond — and vice versa. Santa Clara County’s shoulder season, that stretch from late September into November when mornings are cold and afternoons are still warm, is when this happens most. Households switch back and forth between cooling and heating within the same week, and a thermostat left in the wrong mode is a genuinely common reason a system appears to fail.
Next, check the temperature setting. If the set temperature is lower than the current room temperature and you’re asking for cooling, the system has no reason to run. Bump the target temperature a few degrees in the right direction and wait a minute or two to see if it kicks on.
Finally, check the power to the thermostat itself. If the display is blank or dim, the thermostat may have lost power — either from dead batteries or a tripped circuit. Replace the batteries if it uses them, and then check your breaker panel before assuming anything is wrong with the system itself.
A thermostat issue won’t always be this simple, but it’s the right place to start because it costs nothing to check and takes less than two minutes.
Yes — and this one surprises a lot of homeowners. A clogged air filter doesn’t just reduce airflow. In some systems, it can trigger a safety shutoff that shuts the entire unit down. The system isn’t broken; it’s protecting itself from overheating. But from the outside, it looks exactly like a system failure.
Pull out the filter and hold it up to a light source. If you can’t see light through it, it’s overdue for a change. In Santa Clara County, this matters more than people realize. During wildfire smoke events — which have become a regular part of late summer and early fall here — filters load up far faster than they do in a typical season. A filter that would normally last two to three months can clog in a matter of weeks during heavy smoke. If your system shut down during or just after a smoke event, the filter is the first thing to check.
A clean filter also helps your system run more efficiently when it does start back up. It’s a small thing with a real impact on both reliability and energy use.
If you change the filter and the system still won’t run, reset the system at the thermostat — turn it off, wait a few minutes, and turn it back on. Some systems need a short reset after a safety shutoff before they’ll restart. If it still doesn’t respond, that’s a sign something else is going on and it’s time to call.
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The thermostat and filter cover the majority of false-alarm calls. But there are three more checks worth running before you reach for the phone — especially if you’re heading into the heating season and want to make sure your system is actually ready.
These next checks take a few more minutes, but they’re the ones that cross the line from comfort into safety. A system that won’t cool your home is an inconvenience. A furnace with a blocked vent or a home without working carbon monoxide detectors is a different kind of problem entirely.
If the thermostat looks fine and the filter is clean, head to your breaker panel. HVAC systems typically run on a dedicated circuit, and that circuit can trip without any obvious cause — a brief power fluctuation, a momentary surge, or a system that worked harder than usual during a heat event. Find the breaker labeled for your air conditioner or furnace, confirm it’s fully in the “on” position, and if it’s tripped, reset it once. If it trips again immediately or within a short time, stop. A breaker that won’t stay on is telling you something is wrong electrically, and that’s not a DIY fix.
While you’re outside checking the outdoor unit, look at the area around the condenser. Overgrown shrubs, accumulated debris, or anything sitting against the unit can restrict airflow enough to cause the system to shut down on a high-temperature day. The condenser needs clearance on all sides to do its job. Clear anything that’s crowding it and make sure the unit itself isn’t sitting in standing water, which can happen after heavy rain in low-lying parts of Santa Clara County.
Also take a quick look at whether the outdoor fan is spinning when the system is supposed to be running. If the compressor is humming but the fan isn’t moving, that’s often a sign of a failed capacitor — one of the most common HVAC component failures. A capacitor is a relatively straightforward repair, but it does require a licensed technician. Don’t attempt to open the unit yourself.
If you have a gas furnace and it won’t fire up, check the pilot light. Many older furnaces — and there are a lot of them in San Jose’s established neighborhoods and throughout Santa Clara County’s pre-1980s housing stock — use a standing pilot light rather than electronic ignition. If that pilot light has gone out, the furnace simply won’t run. Relighting it is typically straightforward and is described in your furnace’s owner manual, but if you smell gas at any point during this process, stop immediately, leave the house, and call your gas utility. Don’t attempt to relight anything if there’s a gas odor.
The carbon monoxide detector check is the one most homeowners skip, and it’s the most important safety step on this list. Carbon monoxide is colorless and odorless. A furnace with a cracked heat exchanger, a blocked flue, or a venting issue can produce dangerous levels of CO without any visible sign that anything is wrong.
Before you run your furnace for the first time each fall, test every CO detector in your home. Replace batteries if needed. If a detector is more than five to seven years old, replace the unit. This isn’t an HVAC check in the traditional sense, but it’s directly connected to whether your heating system is safe to operate — and it takes about five minutes.
If you’ve worked through all five of these checks and your system still isn’t running, you’ve done everything you reasonably can on your own. At that point, the problem is something a licensed technician needs to diagnose — and waiting isn’t going to make it better.
Running through these five checks isn’t about avoiding professional help — it’s about making sure you actually need it before you call. If you do need it, you’ll know. And when that moment comes, the right move is to call a technician who is licensed under California’s C-20 contractor requirement, bonded, and genuinely available when you need them.
Santa Clara County’s summers are getting hotter. ClimateCheck projects that by 2050, residents here will experience close to three times as many days above 92°F as they did in 1990. That’s not a distant forecast — it’s a reason to know who you’re going to call before the next heat wave hits, not during it.
We’ve been serving San Jose and Santa Clara County since 1985. We’re available 24/7, including weekends and holidays, and we offer free estimates on every job. If you’ve worked through this checklist and still need help, give us a call — we’ll tell you honestly what’s going on and what it takes to fix it.
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