Air Conditioning Not Cooling? Common Reasons and How to Fix It
If your air conditioning is on but not cooling, common causes include a dirty air filter, incorrect thermostat settings, a blocked outdoor unit, a tripped breaker, or low refrigerant. You can often fix the first few in minutes. Refrigerant and mechanical faults need a certified engineer and an air conditioning repair.
An air conditioner that runs but won't cool is one of the most common problems we're called out for, especially in summer. The reassuring part is that many of these cases come down to simple, fixable issues rather than a major fault.
In this guide, we explain the quick checks you can safely carry out yourself, the faults that need a professional, and why it pays to act quickly rather than push a struggling system to keep running.
What quick checks can I do myself when my AC isn't cooling?
Before booking a repair, work through these checks in order. Each takes only a few minutes and often restores cooling without a call-out.
Check the thermostat mode and settings: Make sure it's set to "cool", not "fan" or "heat", and that the target temperature is a few degrees below the current room temperature. On "fan", the system circulates air without cooling it, a common false alarm. Replace the batteries in the remote or controller if it's unresponsive.
Check and clean the air filter: A clogged, dirty filter is the leading cause of weak cooling. It restricts airflow, so the system works harder and delivers less cold air. Remove the filter, rinse it under warm water if it's dusty, let it dry fully, and refit it. In heavy use, do this every few weeks.
Clear the outdoor unit: The outdoor condenser releases the heat your system removes from indoors. If leaves, grass, or debris smother it, it can't shed that heat and cooling suffers. Switch off the power, clear the area around it, and gently brush debris from the fins.
Check for a tripped breaker: If part of the system has no power, look at your consumer unit for a breaker that has flipped off, and reset it once. If it trips again immediately, leave it off and call us; repeated tripping signals an electrical fault.
Close windows and doors:Â It sounds obvious, but an open window or door lets warm air in and cooled air out, so the room never reaches temperature. Close everything in the room you're cooling and keep sun-facing blinds shut.
If cooling returns after these checks, it's worth booking a service to confirm all is well. If it doesn't, the fault most likely needs an engineer.
Which air conditioning faults need a professional engineer?
Some faults go beyond safe DIY territory because they involve refrigerant and electrical components, or because getting them wrong can cause more damage. If you've completed the checks above and your system still won't cool, one of the following is the likely cause.
Low or leaking refrigerant:Â Refrigerant is the fluid that absorbs heat and cools the air. It isn't "used up", so low levels almost always mean a leak. By law, a qualified, refrigerant-certified engineer must handle refrigerant; it is not a DIY fix.
A faulty compressor: The compressor is the heart of the system. When it struggles, you'll typically get warm air, unusual noises, or a unit that trips the breaker.
A failed fan motor or capacitor: If the indoor or outdoor fan isn't spinning, the system can't move air or release heat. A worn capacitor is a frequent, engineer-fixable cause.
A frozen evaporator coil: Â Restricted airflow or low refrigerant can cause the indoor coil to ice over. You'll see ice on the pipework and little or no cool air. Switch off and let it defrost before an engineer investigates.
A blocked condenser coil: Over time, the outdoor coil clogs with dirt, reducing its ability to release heat. Professional cleaning restores efficiency.
Drainage faults:Â A blocked condensate drain can cause water to leak from the indoor unit or trigger a safety shut-off. Clear this properly to prevent damage.
Here's how these faults usually show up and why each is an engineer's job:
Fault | What you'll notice | Why it needs an engineer |
Low or leaking refrigerant | Warm air, weak cooling, possible ice | Refrigerant handling is legally restricted to certified engineers |
Faulty compressor | Warm air, noises, breaker trips | Central component needing specialist diagnosis |
Failed fan motor/capacitor | Fan not spinning, no airflow | Electrical repair and correct part matching |
Frozen evaporator coil | Ice on indoor coil or pipes, no cool air | Underlying cause (airflow or refrigerant) must be diagnosed |
Blocked condenser coil | Cooling weakens gradually | Professional coil cleaning |
Drainage fault | Water leaking indoors, shut-offs | Safe drain clearing to prevent damage |
Our certified engineers diagnose and repair all of these across domestic and commercial systems, on all major brands.
Why does running a faulty air conditioner make things worse?
It's tempting to keep a struggling unit running for whatever cooling you can get, but this often backfires. A system that is low on refrigerant or has a frozen coil is already under strain, and forcing it to continue can turn an affordable repair into a major one.
Running with low refrigerant, for instance, makes the compressor, the most expensive component, work far harder, risking failure. A frozen coil left running can lead to water damage and further icing.
If your unit is blowing warm air, iced up, leaking, or tripping the breaker, the safest and most cost-effective move is to switch it off and call an engineer. This is also where the 3-minute rule matters: after any shut-off, wait at least three minutes before restarting, so refrigerant pressure can equalise, and the compressor isn't put under unnecessary strain.
When should I call a professional?
Call us if your air conditioning still isn't cooling after the DIY checks, or if you notice any of the engineer-level signs above: warm air only, ice, leaks, repeated tripping, or unusual noises.
We provide air conditioning repairs for homes and businesses across Leicester and the surrounding areas, with certified engineers who work on all major brands. Booking early in a busy summer period gives you the best chance of a prompt visit.
When you contact us, it helps to have your unit's make to hand and a short description of the symptoms. A regular air conditioning service is also the most reliable way to prevent "not cooling" problems in the first place. Our guide on whether you should service your air con explains why.
What to check and when to call us
Start with the easy wins:Â set the thermostat to "cool", clean the filter, clear the outdoor unit, reset a tripped breaker, and close windows and doors.
Warm air, ice, leaks, or tripping usually mean a refrigerant, compressor, fan, coil, or drainage fault an engineer's job.
Don't force a faulty unit to run; it risks turning a small repair into a costly one.
Remember the 3-minute rule before restarting after any shut-off.
Servicing prevents most "not cooling" problems; the cheapest fault is the one that never happens.
Book an air conditioning repair in Leicester
For expert air conditioning installation, servicing, and repairs across Leicester and the surrounding areas, contact Leicester Air Conditioning Services Ltd today. Our showroom is at 8 Hannah Parade, Stonehill Ave, Birstall, Leicester, LE4 4JE, and our registered office is at 51 Verdale Avenue, Leicester, England, LE4 9TH. Call us on 0116 4030540. With 150+ five-star reviews, our certified engineers are trusted throughout Leicester, Nottingham, Derby, and beyond for reliable, energy-efficient solutions.
Frequently asked questions
Why is my air conditioning running but not cooling?
If your unit is running but not cooling, the usual culprits are a dirty filter, incorrect thermostat settings, a blocked outdoor unit, or low refrigerant. Start with the filter and thermostat, then check the outdoor unit. If it still blows warm air, you likely need an engineer.
Why is my air conditioner not blowing cold air?
Warm air often points to low or leaking refrigerant, a frozen evaporator coil, or a failing compressor. It can also be as simple as the thermostat being set to "fan" rather than "cool". Check the settings first; if the air still isn't cold, book a repair.
What is the most common cause of AC failure?
The most common cause is poor airflow from a dirty or clogged filter, which strains the system and reduces cooling. Left unaddressed, restricted airflow can freeze the evaporator coil and lead to bigger faults. Regular filter cleaning and servicing prevent most failures.
What are the most common air conditioning problems?
The most common problems we see are refrigerant leaks, clogged filters, thermostat faults, blocked airflow, faulty compressors or fan motors, frozen coils, and drainage issues. Many begin small and worsen under heavy summer use. Routine servicing catches the majority before they stop your system from cooling.
Should I turn my air conditioning off if it's not cooling?
Yes. If your system is blowing warm air, icing up, or leaking, switch it off. Running a faulty unit can turn a minor issue into an expensive compressor or coil failure. Turn it off, carry out the basic checks, and call an engineer if it still won't cool.
What is the 3-minute rule for air conditioners?
The 3-minute rule means waiting at least three minutes after your AC switches off before turning it back on. The pause lets refrigerant pressure equalise, so the compressor doesn't restart under strain. Many modern units enforce this automatically, but apply it yourself after a breaker reset or power cut.
What should I do if the AC is not working at home?
Check that the thermostat is set to cool, inspect and clean the filter, clear the outdoor unit, and reset a tripped breaker once. Close windows and doors to keep cool air in. If these steps don't restore cooling, contact us to arrange a repair.
