It is 98 degrees in Houston. Your air conditioner is running. You can hear it working. But the air coming from your vents feels weak and warm. You walk over to the indoor unit and find ice on it.

That ice is a warning. It usually points to the evaporator coil, the part of your system that pulls heat out of your indoor air. When that coil gets dirty or freezes, your whole system stops cooling the way it should. Most homeowners in Houston never see this part of their AC. So the signs are easy to miss until the house gets hot.

Below, you'll find the clearest signs of a dirty or frozen evaporator coil, what causes them, and what to do the moment you spot ice. We start with what the coil does and why it matters so much here. Then we walk through the warning signs, the causes behind them, and the steps to take before your system gets damaged.

Watch for these seven warning signs:

• Weak or warm airflow — your AC runs, but the vents barely blow
• Visible ice or frost on the indoor coil or the refrigerant lines
• Longer run times — the system never reaches the temperature you set
• Higher energy bills with no change in how you use your AC
• Water around the indoor unit from a melting coil or a clogged drain
• Musty or sour smells coming from your vents
• Sticky, humid air indoors even while the system runs

If you see ice, turn your system off right away. Running a frozen AC can damage the compressor.

Your evaporator coil sits inside your home. You will find it in the air handler or mounted above the furnace. It is not the unit in your yard. That outdoor box is the condenser, and it does a different job.

The coil looks like a metal grid with rows of thin fins. Many are shaped like the letter A. Cold refrigerant flows through the tubing inside it.

Here is how it cools your house. Your blower pulls warm air from your rooms and pushes it across the coil. The refrigerant absorbs the heat from that air. The cooled air then travels back through your vents.

That whole process depends on air touching the coil surface. If the coil cannot absorb heat, nothing else in your system can cool your home.

Here is what each sign looks like in a real Houston home.

1. Weak or warm airflow — Hold your hand near a vent. The air feels soft instead of strong. Some rooms never cool down, and upstairs is often the worst. Your system is running, but the cool air is not arriving.

2. Visible ice or frost — Look at the indoor unit. You may see white frost on the coil fins. You may also see ice on the copper refrigerant lines. This is the most obvious sign and the most urgent one.

3. Longer run times — Your AC used to cycle on and off. Now it runs and runs. The thermostat says 74, but the house sits at 79. The system is working, but it cannot reach your setting.

4. Higher energy bills — Your habits did not change. Your bill went up anyway. A dirty coil forces your system to run longer to do the same job. You pay for that extra runtime every month.

5. Water around the indoor unit — Your AC pulls moisture from the air and drains it away. When a frozen coil thaws, that water has to go somewhere. If the drain cannot keep up, it pools around the unit.

6. Musty or sour smells — A damp, dirty coil grows mold and bacteria. That smell travels through your ducts and into your rooms. You notice it most when the system first kicks on.

7. Sticky, humid air indoors — Your thermostat reads fine, but the air feels heavy. A dirty coil cannot pull moisture out of your air. In Houston humidity, this one gets uncomfortable fast.

These two problems are related, but they are not the same thing. Knowing which one you have helps you act faster.

A dirty coil builds up slowly. Dust settles on the fins over months or years. Your cooling gets a little weaker each season. There is no single dramatic moment. Most homeowners never notice until a technician points it out.

A frozen coil is sudden and obvious. You see ice. Your cooling drops off hard or stops completely. It demands attention today, not next month.

Here is how the two connect. Dirt on the fins blocks airflow across the coil. With less warm air passing over it, the coil gets colder than it should. Moisture in the air then freezes on the fins instead of draining away.

Ice makes the problem worse. The frozen layer blocks even more airflow. That freezes more moisture, which blocks more air. The system spirals downward on its own.

One important caution. Ice does not always mean dirt. Low refrigerant from a leak can freeze a clean coil. So can a weak blower or a thermostat that never shuts the system off. Same symptom, different repair.

That is why we thaw the coil first, then look for the reason. The ice is the symptom, not the diagnosis.

Almost every frozen coil traces back to one thing: air is not moving across it the way it should. Here is what blocks it.

A dirty air filter — This is the most common cause and the easiest one to prevent. A clogged filter chokes off airflow to the coil. It also lets more dust slip past and settle on the fins. The U.S. Department of Energy notes that replacing or cleaning your air filter can lower your air conditioner's energy use by 5 to 15 percent.

Blocked return vents — A couch, a rug, or a stack of boxes over a return starves your coil. Walk your house and check that every return is clear.

The coil itself is dirty — Dust on the fins acts like a blanket. It insulates the coil and blocks heat from reaching the refrigerant.

Low refrigerant — A leak makes the coil run colder than it was built to run. This needs a real repair, not a top-off. Adding refrigerant to a leaking system just delays the problem.

A weak or failing blower — If the blower cannot push enough warm air across the coil, the coil gets too cold.

A thermostat that never cycles off — If your system runs all night without shutting down, you may find ice in the morning.

Houston makes all of this worse. Our air holds a lot of moisture, so more water hits the coil every hour it runs. When airflow drops here, the coil freezes faster than it would in a dry climate. Our long cooling season also loads up filters and coils sooner than most of the country.

Ice that keeps coming back means the cause is still there. Our AC repair services find the reason, not just the ice.