Air conditioning troubleshooting checklist

Checks anyone can safely do

• Check the remote control has working batteries and is set to the correct mode, not accidentally left on fan-only or heat.
• Check the isolator switch near the outdoor unit and any relevant circuit breaker or fuse in the consumer unit haven't tripped.
• Check the indoor unit's filters aren't visibly clogged with dust or fluff, since blocked filters restrict airflow and can trigger the unit to shut down.
• Check nothing is blocking the outdoor unit, such as overgrown plants, bins, snow or fallen leaves restricting airflow around it.
• Check the set temperature is actually below room temperature in cooling mode, or above it in heating mode, since it sounds obvious but is a common cause of 'it's not doing anything' calls.
• Check for any error code shown on the indoor unit's display or flashing lights, and note it down exactly before calling for help.

What not to attempt yourself

Air conditioning systems contain refrigerant under pressure and mains electrical connections, both of which are legally and practically off-limits to untrained hands. You should never remove the indoor or outdoor unit's outer casing, top up or handle refrigerant, or work on the electrical isolator or wiring behind it.

If a check involves anything beyond the remote, filters or an external obstruction, it's a job for a qualified engineer. Attempting electrical or refrigerant work yourself can void your warranty, breach F-Gas regulations, and in some cases create a genuine safety risk.

Symptom to likely cause

• No power at all: check the isolator and consumer unit first, then suspect a wiring fault or failed control board if both are fine.
• Runs but doesn't cool: often low refrigerant, a blocked filter, an oversized room for the unit, or an outdoor unit blocked by debris.
• Blows warm air only: frequently a mode setting issue, but can also indicate a refrigerant or compressor fault if the setting is correct.
• Water dripping indoors: usually a blocked or disconnected condensate drain, or a full condensate pump on units with one fitted.
• Ice forming on the indoor coil: commonly low refrigerant, restricted airflow from dirty filters, or running in cooling mode at very low outdoor temperatures.
• Unit trips the electrics repeatedly: suggests an electrical fault and should not be reset repeatedly, since this can cause further damage.
• Unusual noise on start-up or shutdown: sometimes normal expansion and contraction of casing parts, but persistent grinding or rattling needs investigating.

When to call an engineer straight away

Some symptoms warrant a call without further home troubleshooting: a burning smell, visible sparking, water pooling near electrical fittings, or a unit that trips the electrics every time it's switched back on. These aren't situations to keep testing yourself, even with the safe checks above.

For anything less urgent, working through the checklist first genuinely helps. Many call-outs are resolved by checking a filter or isolator, and knowing you've already ruled those out lets an engineer get straight to diagnosing the real fault when they arrive.

Getting it looked at properly

If your own checks haven't resolved the problem, or you're dealing with any of the more serious symptoms above, it's worth booking a proper diagnostic visit rather than guessing further. Recurring faults are also a good sign that a system hasn't been serviced recently, since many of the common causes above are exactly what routine servicing is designed to catch early.

We cover home and business air conditioning faults across Hertfordshire, Bedfordshire, Buckinghamshire, Essex and north-west London. If your checklist hasn't turned up an answer, get in touch for a free quote and we'll get an engineer out to diagnose it properly.