When your AC runs but the air isn't cold, the cause ranges from a 5-minute filter swap to a failing compressor. Here's how to narrow it down.
An AC that's running but not cooling is one of the most common summer service calls in the Denver metro. The good news: a large share of causes are simple and inexpensive. Here's what to check, in order of likelihood.
1. A dirty or clogged air filter
This is the #1 cause of weak cooling, and it's the easiest to fix. A clogged filter restricts airflow across the evaporator coil, which means less cool air reaches your rooms and can eventually freeze the coil. Check your filter first — if it's gray and opaque, swap it. This alone resolves a surprising number of 'AC not cooling' calls.
2. A frozen evaporator coil
If airflow is restricted (usually by a dirty filter) or refrigerant is low, the evaporator coil can freeze over. A frozen coil can't absorb heat, so you get warm air. If you see ice on the refrigerant line or the indoor unit, turn the AC off and let it thaw, then address the root cause — filter or refrigerant.
3. Low refrigerant / a refrigerant leak
Refrigerant doesn't get 'used up' — if your system is low, there's a leak. Low refrigerant means the system can't absorb enough heat to cool your home. This requires professional diagnosis to find and seal the leak, then recharge the system to the correct level.
4. Thermostat issues
If your thermostat is set to 'heat' instead of 'cool,' set to a temperature above the room, or has dead batteries, the AC won't cool properly. Check the mode, setpoint, and batteries before calling. A miscalibrated or failing thermostat may also need replacement.
5. A tripped or blown breaker
Your AC has separate indoor and outdoor components, each on its own circuit. If the outdoor unit's breaker trips, the indoor fan still runs — blowing room-temperature air. Check your breaker panel and reset any tripped breakers. If it trips again, there's an electrical or compressor issue.
6. A dirty outdoor condenser unit
The outdoor unit releases heat from your home. If the fins are clogged with dirt, leaves, or debris, it can't release heat efficiently and cooling suffers. Keep the area around the unit clear and have it cleaned during seasonal maintenance.
7. A failing compressor
The compressor is the heart of the AC system. If it's failing, the system may run but produce no cooling at all. This is the most serious cause and often means replacement is more cost-effective than repair, especially on older units.
When to call a professional
If you've checked the filter, thermostat, and breakers and your AC still isn't cooling, it's time for a diagnostic visit. Low refrigerant, frozen coils, and compressor issues all require professional tools and expertise. We service all major brands across Lakewood and the west Denver metro.
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