Air Con Not Working? A Simple Troubleshooting Guide

There’s nothing worse than your air conditioner giving up the ghost just as a heatwave hits. Whether it’s blowing warm air, making scary noises, or dripping water where it shouldn’t, a broken AC can quickly turn your cool sanctuary into a sauna. Don’t sweat it though! Often, the problem is something simple you can fix yourself.

This guide is your first-aid kit for a faulty air con. We’ll walk you through the most common reasons your air con might not be working properly in the UK and show you some easy troubleshooting steps you can safely try before calling in the professionals. From checking filters to thermostat settings, these simple fixes could get your cool air flowing again in minutes.

Step 1: The Basic Checks (Do These First!)

Before you dive deeper, let’s cover the simple stuff that catches people out all the time:

• Check the Power: Is the unit actually plugged in and switched on at the wall? Has the switch for the air con tripped in your main fuse box? Flick it back on if it has.
• Check the Thermostat: Make sure it’s set to ‘COOL’ mode (often a snowflake symbol ❄️) and the target temperature is set lower than the current room temperature. If it’s battery-powered, try replacing the batteries.
• Check the Remote: Obvious, but easily overlooked! Try fresh batteries. Make sure nothing is blocking the signal between the remote and the AC unit.

Problem: My Air Con Isn’t Cooling Properly

If the basic checks don’t work, the most likely reason your AC isn’t cooling is blocked airflow.

[1] Check the Air Filter (The #1 Culprit!)

Your AC unit has filters inside the indoor unit that catch dust and debris. If these get clogged, air can’t flow through properly, and the cooling performance drops dramatically. It’s the single most common cause of cooling problems.

How to Check (Safely): Switch off the power to the unit first. Then, check your manual to see how to access the filters (usually behind a front panel that clips or slides off). Slide them out – if they look dusty or clogged, they need cleaning or replacing. We cover how to clean them further down!

[2] Check the Outdoor Unit (Condenser)

The big box outside needs to breathe! It draws air through its side vents to get rid of the heat removed from your room. If these vents are blocked, it can’t cool efficiently.

How to Check (Safely): Again, **switch off the power** first. Go outside and look at the unit. Are there leaves, overgrown plants, rubbish bins, or anything else blocking the vents around the sides? Gently clear any obstructions away, ensuring there’s at least a foot or two of clear space around it.

Problem: My Air Con is Making Strange Noises

Unusual noises are a sign something isn’t right. If you hear loud banging, grinding, or squealing, switch off the unit immediately at the fuse box and call a professional. Don’t try to fix these yourself.

• Buzzing/Humming: This often points to an electrical issue inside the unit. Action: Call an engineer.
• Rattling: Could be loose screws on the casing, debris caught inside, or a problem with the fan motor. Action: Call an engineer.
• Hissing/Gurgling: This can indicate a refrigerant leak, which needs professional attention. Action: Call an engineer.

An Engineer’s View

“Listen to your air con – it’ll often tell you what’s wrong! While a gentle hum is normal, any sudden rattles, bangs, or hisses are signs to switch it off immediately. Trying to run it ‘just a little longer’ when it sounds sick can turn a simple repair into needing a whole new unit. If in doubt, power it down and call a pro.” – Rhys Powell, Certified F-Gas Engineer

Problem: My Air Con is Leaking Water

Air conditioners naturally produce condensation (water) as they cool the air. This water is supposed to drain away harmlessly outside through a small pipe. If it’s leaking inside, it usually means something is blocked.

[1] Check the Drain Pipe

This is the most common cause. The small plastic pipe leading outside can get blocked by algae, sludge, or insects.

How to Check (Safely): If you can safely see and reach the end of the drain pipe outside, check if it’s dripping. If it’s dry on a hot day when the AC has been running, it might be blocked. Look for any obvious kinks or blockages. Sometimes a gentle prod with something flexible (like a net curtain wire) can clear a simple blockage at the very end.

[2] Frozen Coils

If the air filter is severely blocked, the super-cold coils inside the unit can actually freeze over. When the ice melts, it can overwhelm the drain system, causing leaks.

How to Fix (Safely): Switch the unit off completely and let it defrost for several hours. Then, thoroughly clean or replace the air filter before switching it back on.

Problem: My Air Con Smells Musty

A damp, musty smell usually means mould or bacteria has built up inside the unit due to moisture. Start by cleaning the air filter thoroughly (see below). If the smell persists after cleaning the filter, it likely needs a professional deep clean and service.

How to Clean Your AC Filter Safely

Cleaning your filters regularly (every few weeks during heavy use) is the best way to keep your AC running efficiently and prevent problems.

1. Switch it Off: Turn off the power at the fuse box. Safety first!
2. Locate the Filter(s): Open the front panel of the indoor unit (check your manual if unsure).
3. Remove and Clean: Slide the mesh filters out. You can usually vacuum them gently or rinse them under lukewarm water (don’t use hot water or soap).
4. Dry Fully: Let the filters air dry completely. Don’t put them back wet!
5. Pop it Back In: Slide the clean, dry filters back in and close the panel.

When to Stop Troubleshooting and Call an Engineer

DIY checks can solve simple issues, but know when to call in the experts. Contact a qualified air conditioning engineer if:

• You’ve checked the power basics and it still won’t turn on.
• The unit is making loud banging, grinding, or squealing noises.
• You suspect a refrigerant leak (hissing sound, oily residue near pipes, not cooling well).
• It keeps leaking water even after checking the drain and cleaning filters.
• Error codes are showing on the unit’s display.
• You’re simply not comfortable performing any of the checks yourself.

A professional call-out fee or diagnostic charge in the UK typically ranges from £80-£150, plus the cost of any parts and labour for the repair itself.

Keeping Your Cool

Often, getting your air con working again is as simple as cleaning a filter or checking the thermostat. By running through these basic troubleshooting steps, you can often save yourself the cost and hassle of a call-out.

However, always prioritise safety. If you hear scary noises, suspect leaks, or just aren’t sure, switch it off and leave it to the professionals. Sometimes, an older unit might just be beyond economical repair.

Time for an Upgrade?

If your AC unit is old, inefficient, or constantly causing problems, it might be time to consider a replacement. Get a fixed-price quote for a brand new, energy-efficient air conditioner from WarmZilla in minutes. We’ll help you find the perfect unit for your home, installed by one of our certified engineers.