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Troubleshooting Guide: What to Do If Your Wall Unit Isn’t Cooling (or Heating)

Hi, it’s Samantha! If your through-the-wall air conditioner or AC/heat combo unit suddenly isn’t working right, take a deep breath. Don’t panic, and definitely don’t start Googling for a new $1,000 replacement just yet. Most of the time, the problem is something small — a dirty filter, a tripped breaker, or even the thermostat setting. I’ll walk you through a step-by-step checklist so you can figure out what’s wrong, try the simple fixes first, and know when it’s time to call in a pro.

Part 1: Before You Panic — The Quick-Fix Checklist

Let’s start simple. Sometimes the “broken” AC just needs a little nudge.

Check the Power Supply
- Is the unit plugged in fully?
- Did the circuit breaker trip? Reset if necessary.
- If on a power strip, plug directly into the wall (ACs draw too much for strips).
Look at the Thermostat Setting
- Is it actually set to “Cool” or “Heat”?
- Try lowering the temp 5 degrees for cooling (or raising it for heating).
- If digital, check batteries in the remote or thermostat.
Inspect the Air Filter
- Dirty filters block airflow.
- If clogged, clean or replace and try again.
Check the Mode & Fan Settings
- “Fan only” mode won’t cool or heat.
- Make sure you’re in the correct mode.

👉 Samantha’s reminder: I know it sounds obvious, but you’d be surprised how many “broken AC” calls end with the tech flipping the mode switch.

Part 2: Why Isn’t It Cooling?

If your wall AC is blowing warm air, here are the most common culprits:

Dirty filter or coils: Air can’t flow properly, so cooling power drops.
Blocked vents: Furniture, curtains, or outside obstructions reduce airflow.
Low refrigerant: Leaks or low charge = weak cooling. (Pro job.)
Frozen evaporator coil: Often caused by dirty filters or running the unit on very low temperatures for too long.
Faulty thermostat: Misreads the room temp.
Electrical issues: Loose connections or a tripped breaker.

According to Energy.gov , clogged filters and dirty coils are the top reasons ACs lose cooling efficiency.

Part 3: Why Isn’t It Heating?

If your wall unit has a built-in heater or heat pump and it’s not warming the room:

Wrong mode selected: Double-check the remote or panel.
Outside temperature too low: Heat pumps lose efficiency in very cold weather (below 30°F); many units struggle.
Dirty filter: Blocks airflow, just like with cooling.
Thermostat calibration off: Tells the unit the room is warmer than it is.
Heating element failure (in electric units): Needs replacement.
Reversing valve stuck (in heat pumps): A common pro repair.

👉 Samantha’s tip: If your unit heats fine in mild weather but struggles in freezing temps, it’s not broken — it’s just the limit of that system type. A backup heater may be necessary.

Part 4: DIY Troubleshooting Steps

Here’s the order I recommend tackling problems:

Step 1: Power Cycle the Unit

Turn it off, unplug it, wait 5 minutes, then plug it back in.
Electronics sometimes just need a reset.

Step 2: Clean the Filter

Wash or replace.
Run the AC again and see if cooling/heating improves.

Step 3: Inspect the Coils

Remove the front cover and check for dirt.
If dusty, vacuum with a brush attachment.
For heavy buildup, use foaming coil cleaner (Family Handyman ).

Step 4: Check Airflow

Look outside: is the condenser vent blocked by leaves, weeds, or furniture?
Inside: Are supply vents open and unobstructed?

Step 5: Defrost Frozen Coil

If you see ice, turn the unit to “Fan Only” for a few hours.
Check the filter and airflow before running “Cool” again.

Step 6: Verify Thermostat Accuracy

Use a separate thermometer to compare.
If the AC says it’s 72°F but the room is 78°F, the thermostat may need recalibration.

👉 Samantha’s hack: A small $10 digital thermometer is a lifesaver for checking thermostat accuracy.

Part 5: When to DIY vs. When to Call a Pro

Safe DIY Fixes:

Cleaning/replacing filters
Vacuuming coils and vents
Resetting breakers or power cycling
Removing ice from a frozen coil

Call a Pro If:

Refrigerant is low (requires licensed handling)
Electrical smells, sparks, or burnt wires
Loud grinding or buzzing noises
Heating element or reversing valve issues
Unit is under warranty (don’t void it!)

According to Consumer Reports , refrigerant and electrical issues are not DIY jobs — they need certified technicians.

Part 6: Cooling Troubleshooting Flowchart (Quick Guide)

Unit is on but not cooling?

Check filter → Clean/replace.
Check mode → Must be on “Cool.”
Check thermostat → Lower 5 degrees.
Look for ice → Defrost if frozen.
Still warm air? Likely refrigerant or compressor → Call pro.

Part 7: Heating Troubleshooting Flowchart

Unit is on but not heating?

Mode → Set to “Heat,” not “Fan.”
Thermostat → Raise 5 degrees.
Filter → Clean/replace.
Outside temp → Below 30°F? The heat pump may struggle.
Still no heat? Likely heating element or reversing valve → Call pro.

Part 8: Preventing Problems Before They Start

Here’s how to stop the “uh-oh, it’s not cooling” moment:

Monthly filter cleaning during heavy use.
Annual coil cleaning in spring.
Check seals around the sleeve for drafts and leaks.
Don’t overload circuits — AC should have its own outlet.
Run “Fan Only” mode occasionally to keep air circulating and reduce coil freeze.

EnergyStar emphasizes routine filter swaps and coil cleaning as the simplest way to extend lifespan and prevent efficiency drops.

Part 9: Real-Life Stories

Kayla’s “No-Cool” Scare: Thought her Friedrich was dead in July. Turns out the filter hadn’t been cleaned in months. Ten minutes later, the problem was solved.
Dan’s Heating Woes: His wall heat pump blew cold air in January. Tech explained that at 20°F, the unit just couldn’t keep up. Dan added a space heater for backup.
Marcus the DIYer: Noticed water dripping inside. Instead of panicking, he checked the drain hole — clogged with debris. Cleared it, no more leaks.
Lisa’s Lesson: Ignored rattling for weeks. By the time she called a tech, the fan motor had burned out. $300 repair that could’ve been avoided with early action.

Part 10: Troubleshooting Myths Busted

❌ “If it’s blowing air, refrigerant must be fine.” Wrong — low refrigerant still blows air, just not cold.
❌ “More BTUs = better.” Oversized units have short cycles and don’t dehumidify properly.
❌ “No heat means it’s broken.” In sub-freezing weather, heat pumps naturally lose efficiency.
❌ “ACs don’t need cleaning.” Dirt is the #1 cause of failure.