AC Repair
Emergency HVAC
By :
TL;DR
AC Troubleshooting
When your AC runs but the air feels warm, the cause usually traces back to one of seven common culprits. This guide walks through each, from a dirty filter or frozen coil to a clogged condenser, low refrigerant, leaky ducts, and compressor problems, with clear guidance on what you can fix yourself and what needs a professional.
It's 95 degrees in late July, the monsoon humidity has rolled in over the Front Range, and the air drifting out of your vents feels closer to a warm breeze than cold air conditioning. When your AC is not blowing cold air, the problem usually traces back to one of seven common culprits, and the good news is that several of them you can diagnose (and sometimes fix) yourself before you ever pick up the phone.
Denver's climate puts unusual stress on cooling equipment. At 5,280 feet, thinner air changes how your system rejects heat, cottonwood fluff and dust coat outdoor condensers every spring, and sudden monsoon humidity can freeze up a coil that ran fine the week before. So while the seven reasons below apply to any home, we've added the Mile High details that actually matter for homeowners from Lakewood to Aurora.
Here's the quick scannable list of what's most likely going on:
1. Dirty or clogged air filter — restricts airflow (easy DIY fix)
2. Thermostat set wrong or failing — wrong mode or dead batteries (easy DIY fix)
3. Frozen evaporator coil — ice blocks heat transfer (DIY thaw, then investigate)
4. Dirty outdoor condenser coil — can't dump heat (rinse DIY, deep clean by pro)
5. Low or leaking refrigerant — system can't cool (licensed tech only)
6. Leaky or disconnected ducts — cold air escaping (pro inspection)
7. Failing compressor or capacitor — the heart isn't pumping (licensed tech only)
Let's walk through each one so you know exactly what to check, what's safe to handle on your own, and when it's time to call in a professional.
Start With the Easy Wins: Filter and Thermostat
Before assuming the worst, rule out the two most common — and most fixable — causes. A surprising number of "my AC stopped cooling" calls we get across Westminster and Thornton come down to one of these.
Reason 1 — A dirty or clogged air filter. Your air filter is the first thing to check, and it's the number-one reason an otherwise healthy system stops blowing cold air. When the filter clogs with dust, pet hair, and Denver's notorious springtime cottonwood and pollen, airflow across the indoor coil drops. Less airflow means less heat gets carried away, your home stays warm, and — worse — the coil can freeze over (more on that below). Pull the filter out and hold it up to a light. If you can't see through it, replace it. During cottonwood season in May and June, you may need to swap filters monthly instead of every 90 days. This is a safe, five-minute DIY fix and the cheapest insurance you can buy. Our complete guide to changing and maintaining air filters walks through sizing and MERV ratings.
Reason 2 — The thermostat is set wrong or failing. It sounds obvious, but confirm the thermostat is set to COOL (not just "fan" or "heat") and that the temperature is set below the current room temperature. A thermostat in "fan ON" mode runs the blower continuously without cooling, which feels exactly like warm air from the vents. If the settings look right, replace the batteries — a dying thermostat battery is a common, sneaky cause. Older or failing thermostats can also lose their connection to the system entirely. If yours is more than ten years old, upgrading to a smart model can improve accuracy and save energy; see our roundup of smart thermostats that actually save you money.
When Ice Is the Problem: A Frozen Evaporator Coil
Reason 3 — A frozen evaporator coil. It seems backward, but ice on your AC is a real and common reason it can't cool. The evaporator coil sits inside your air handler, and when warm household air passes over it, refrigerant absorbs the heat. If airflow is restricted (dirty filter) or refrigerant is low, the coil gets too cold, condensation freezes, and a block of ice forms that blocks heat transfer entirely. The result: the system runs hard but blows warm or weakly cooled air, and you may notice water pooling near the indoor unit as the ice melts.
Denver's July and August monsoon humidity makes freeze-ups more likely than you'd expect in a semi-arid climate, because that extra moisture condenses fast on an already-cold coil. If you spot ice, the DIY step is to turn the system OFF at the thermostat, switch the fan to ON to help the coil thaw, and replace a dirty filter. Give it several hours to fully melt. If it freezes again after thawing, that points to low refrigerant or an airflow problem that needs a technician. Frozen coils are one of the 5 signs you need air conditioner repair — don't ignore a coil that keeps icing over.
The Outdoor Unit: A Dirty Condenser Coil
Reason 4 — A dirty outdoor condenser coil. Your outdoor unit (the condenser) is where your system dumps all the heat it pulled from your house. Its coil is wrapped in thin metal fins that need clear airflow to release that heat. In Denver, those fins get caked with cottonwood fluff, lawn clippings, dust, and dryer lint surprisingly fast. When the condenser can't shed heat, the whole cooling cycle backs up and your vents blow warm air.
At our 5,280-foot altitude, thinner air already makes heat rejection less efficient, so a dirty coil hurts Denver systems more than it would at sea level. The DIY fix: turn off power to the unit at the disconnect or breaker, clear away leaves and debris, and gently rinse the fins from the inside out with a garden hose on low pressure. Never use a pressure washer — you'll bend the delicate fins. Keep at least two feet of clearance around the unit. A deeper chemical cleaning of the coil, however, should be left to a technician. This kind of buildup is exactly why annual AC tune-ups are so important in our dusty, high-pollen climate.
Low or Leaking Refrigerant: A Job for the Pros
Reason 5 — Low or leaking refrigerant. Refrigerant is the fluid that actually absorbs and moves heat out of your home. It doesn't get "used up" in a sealed system — if your levels are low, you have a leak. Signs include warm air from the vents, hissing or bubbling sounds, ice on the refrigerant lines or coil, and a system that runs constantly without reaching the set temperature.
This is not a DIY repair. Handling refrigerant requires EPA certification by law, specialized gauges, and leak-detection tools. Simply "topping off" refrigerant without finding the leak is illegal, wasteful, and only delays the failure. Our EPA-certified technicians locate the leak, repair it, and recharge the system to the manufacturer's exact spec — which matters even more at Denver's altitude, where correct charge levels are critical to performance. If you suspect a refrigerant issue, our AC repair service can diagnose and fix it properly. To understand what "cold enough" should feel like, check our explainer on the 20-degree rule for air conditioning.
Leaky or Disconnected Ducts: Cold Air Going Nowhere
Reason 6 — Leaky or disconnected ducts. Sometimes the AC is making cold air just fine — it's simply not reaching your rooms. Ductwork running through hot attics, crawlspaces, and unconditioned basements common in older Denver homes can develop leaks at seams and connections, or a duct can become fully disconnected. The U.S. Department of Energy estimates that typical homes lose around 20–30% of conditioned air to duct leakage. That's cold air you paid to make, leaking into your attic instead of your living room.
You may notice some rooms never cool down while others are fine, weak airflow at certain vents, or a sudden jump in your summer energy bills. Many older homes in neighborhoods like Wheat Ridge and Arvada have aging ductwork that's never been sealed. Spotting and sealing leaks usually requires a professional inspection with the right tools, though you can visually check accessible ducts for obvious disconnections or crushed flex lines. Sealing leaky ducts is also one of the most effective ways to lower your summer energy bills.
The Compressor or Capacitor: When the Heart Fails
Reason 7 — A failing compressor or capacitor. The compressor is the heart of your AC — it pressurizes and circulates the refrigerant that makes cooling possible. The capacitor is the small component that gives the compressor and fan motors the jolt of energy they need to start. When a capacitor weakens or fails, you'll often hear a humming or clicking sound from the outdoor unit while the fan struggles to start, and the system blows warm air. A failing compressor may run loudly, trip the breaker repeatedly, or refuse to start at all.
Both of these are licensed-technician repairs. Capacitors store an electrical charge that can deliver a dangerous shock even when the power is off, and a compressor replacement is a major job that requires evacuating and recharging the refrigerant. If your AC won't even turn on, work through our guide on what to do if your air conditioner won't turn on first, then call a pro. On an aging system in Centennial or Littleton, a failed compressor is often the moment homeowners weigh repair against replacement.
DIY vs. Call a Pro: A Quick Reference
Here's the bottom line on which of these you can safely tackle and which need our team:
Safe DIY: Replacing a clogged air filter, checking and resetting the thermostat (including batteries), thawing a frozen coil with the system off, and gently rinsing the outdoor condenser.
Call a licensed technician: Low or leaking refrigerant, duct sealing, and any compressor or capacitor problem. These involve EPA-regulated refrigerant, high-voltage components, or specialized diagnostics.
A simple rule of thumb: if the fix involves the sealed refrigerant system or electrical components, it's time to call. Trying to handle those yourself risks injury, code violations, and turning a small repair into an expensive replacement.
Your Local Partner for AC Repair in Denver
When your AC is not blowing cold air and the easy fixes don't solve it, you don't have to sweat it out. As a NATE-certified, EPA-certified, and BBB-accredited company with over 16 years serving the Denver Metro Area, MoJo Home Services has diagnosed every cooling problem the Front Range can throw at a system — from cottonwood-clogged condensers to monsoon-season freeze-ups.
Our technicians arrive with the tools to pinpoint the real cause fast, explain your options in plain language, and get cold air flowing again — often the same day. We proudly serve homeowners across Aurora, Lakewood, Westminster, and the surrounding communities with honest, reliable AC repair in Denver.
Don't spend another sweltering afternoon guessing what's wrong. Contact our team today to schedule your AC repair and get your home comfortable again — with the peace of mind that comes from a job done right.
Sources & References
U.S. Department of Energy, Air Conditioner Maintenance, Office of Energy Efficiency & Renewable Energy, Maintaining Your Air Conditioner, 2024
U.S. Department of Energy, Duct Sealing, Office of Energy Efficiency & Renewable Energy, Minimizing Energy Losses in Ducts, 2024
ENERGY STAR, Heating & Cooling Maintenance Checklist, U.S. EPA, 2024
U.S. Environmental Protection Agency, Section 608 Refrigerant Management, EPA Office of Atmospheric Programs, 2024
Air Conditioning Contractors of America (ACCA), Standard 4: Maintenance of Residential HVAC Systems, ACCA, 2023
North American Technician Excellence (NATE), Homeowner Resources: Common AC Problems, NATE, 2023
Need expert AC repair in Denver? MoJo Home Services provides professional heating and cooling services throughout the Denver Metro Area. Contact us at 4000 Newman St, Wheat Ridge, CO 80033 or call (720) 807-4050 for same-day service.
Frequently Asked Questions
Why is my AC running but not blowing cold air?
If your AC is running but the air feels warm, the most common causes are a clogged air filter restricting airflow, a thermostat set to the wrong mode, or a frozen evaporator coil. Less obvious culprits include a dirty outdoor condenser, low refrigerant from a leak, leaky ducts, or a failing capacitor. Start by checking the filter and thermostat — those two fixes solve the majority of warm-air complaints. If the easy checks don't help, you likely have a refrigerant or electrical issue that needs a licensed technician to diagnose safely.
Can I fix my AC not blowing cold air myself?
Some causes are safe DIY fixes: replacing a dirty filter, correcting thermostat settings or batteries, thawing a frozen coil with the system off, and gently rinsing the outdoor condenser with a garden hose. However, low refrigerant, duct sealing, and compressor or capacitor failures should always be handled by a licensed, EPA-certified technician. These involve regulated refrigerant and high-voltage components that can cause injury or expensive damage if mishandled.
Does Denver's altitude affect how my AC cools?
Yes. At 5,280 feet, the thinner air makes it slightly harder for your outdoor condenser to reject heat, so anything that further restricts that process — like a dirty coil caked with cottonwood and dust — hits Denver systems harder than at sea level. Correct refrigerant charge is also especially important at altitude. This is why a unit that's underperforming in Denver often needs professional attention sooner than the same model would elsewhere.
Why does my AC freeze up during Denver summers?
Even in our semi-arid climate, the July and August monsoon season brings spikes of humidity. When that extra moisture meets an already-cold evaporator coil — especially one with reduced airflow from a dirty filter or low refrigerant — condensation freezes into ice that blocks cooling. If your coil keeps freezing after you thaw it and change the filter, it usually signals low refrigerant or an airflow problem that requires a technician.
How often should I change my air filter in Denver?
A standard recommendation is every 90 days, but Denver's environment often demands more frequent changes. During cottonwood and pollen season in May and June, and during wildfire-smoke events in summer and fall, you may need to swap your filter monthly. Homes with pets or allergy sufferers should also check more often. Holding the filter up to a light is a quick test — if you can't see through it, replace it.
When should I call a professional instead of troubleshooting?
Call a licensed technician any time the problem points to the sealed refrigerant system (hissing sounds, persistent ice, recharging) or electrical components (humming, clicking, tripped breakers, a unit that won't start). You should also call if your coil refreezes after thawing, if some rooms never cool, or if your system is running constantly without reaching the set temperature. These signs typically mean refrigerant, duct, or compressor problems that require professional tools and certification.
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