Air Conditioning Won't Turn On? How to Diagnose a Dead Unit
No lights, no beep, no response from the remote — but a completely dead air conditioner is far more often a power or remote problem than a write-off. Here's the order an engineer works through, what's safe to try yourself, and what a repair costs in 2026.
Why your air conditioning won't switch on
Most of the time it's power or the remote, not the unit itself. Check the isolator switch and the consumer unit first — a knocked-off isolator or a tripped breaker takes the whole system down. Then rule out flat remote batteries by pressing the manual button on the indoor unit. If the unit clearly has power but still won't respond, you're usually looking at a control board fault, and that needs an F-Gas certified engineer. One thing not to do: keep flicking a breaker that trips straight back on. That's a fault asking to be found, not a glitch to reset.
The checks an engineer makes, in order
A unit that won't power up narrows down quickly. The first three checks are yours and cost nothing; the rest are electrical and belong to a qualified engineer.
1. Power at the isolator and consumer unit
This is where most "dead" units are found. Every split system has an isolator switch — usually a small rocker or rotary switch on the wall near the indoor unit or beside the outdoor condenser — and it's easily caught by a cleaner, a curtain or a decorator and switched off. Check it's on. Then look at your consumer unit: air conditioning normally sits on its own circuit, so a tripped RCD or MCB, or a blown fuse in the fused spur, will leave it stone dead while the rest of the house runs fine. Reset a tripped breaker once. If it holds, you're done. If it trips again the moment the unit tries to start, leave it off and book an engineer — something on the circuit is faulting and repeated resets can make it worse.
2. The remote, not the unit
A surprising share of "it won't turn on" calls are a flat set of AAA batteries. Swap them, then point the remote at the unit and watch for the beep and the display. If nothing happens, prove the unit itself: lift the front flap of the indoor head and you'll find a small manual button (often marked AUTO or with a hand symbol) that starts the unit without the remote. If it kicks into life on that button, the fault is the remote or its infrared — a replacement handset is cheap. If the manual button does nothing but the isolator is on, move to the next step.
3. Standby, timers and protection lockouts
Modern units go into a protective lockout after certain faults and simply refuse to start, sometimes with a blinking light or a code on the display rather than an obvious error. A weekly timer or sleep schedule can also make a healthy unit look dead at the wrong time of day. Switch the system off at the isolator, wait a full ten minutes, then power back on — that clears many lockouts and lets the control board reboot. If a code appears, note it down; our error codes by brand guide translates the common ones so you know whether it's a sensor, a communication fault or something that needs parts.
4. The indoor control board (PCB)
If the isolator is on, the breaker holds, the remote is fresh and the manual button still gets no display, no beep and no fan, the problem is usually the indoor printed circuit board or its low-voltage transformer. A power surge, a lightning-heavy storm or simply age can take out a board or blow the tiny fuse on it. This isn't a DIY job — the board carries mains voltage on one side and delicate electronics on the other — but it's a routine replacement for an engineer, and on most domestic splits it's economic to repair rather than replace the whole system.
5. The outdoor unit won't start
Sometimes the indoor head lights up and responds but nothing happens because the outdoor condenser never fires. A failed run capacitor, a stuck contactor or a fault on the outdoor board are the usual suspects. You might hear a hum and a click outside with no fan, or complete silence. These are same-day repairs in most cases, and the bill is often mostly the call-out plus a modest part. Don't open the casing yourself: capacitors hold a charge long after the power is off.
6. Age and the replace question
If the unit is well past ten years old, runs on obsolete R22 or an older R410A charge, and now needs a board or an outdoor part, it's worth pausing before you spend. Once a repair quote climbs past roughly half the price of a new system, a new, warrantied unit usually wins. Our repair or replace guide walks through the maths.
What's safe to check, and what isn't
The first three are fine for anyone. Once a cover comes off, you're into mains voltage, stored charge and a sealed refrigerant circuit — that's engineer territory, not a weekend job.
| Check | Safe to do yourself? | Why |
|---|---|---|
| Confirm the isolator switch is on; reset a tripped breaker once | Yes | Nothing to open — a switch on the wall and a lever in the consumer unit |
| Fit fresh remote batteries; try the manual button on the indoor unit | Yes | No tools, no electrics — rules the remote in or out in seconds |
| Power off at the isolator, wait ten minutes, restart | Yes | Clears many protection lockouts and reboots the control board |
| Reset a breaker that trips straight back on, again and again | No | Repeated tripping is a genuine electrical fault; find it, don't force it |
| Open the indoor unit to test the PCB or transformer | No | Mains voltage on the board and delicate electronics — easy to damage or get hurt |
| Open the outdoor unit to test the capacitor or contactor | No | Capacitors store a charge after power is off; live terminals inside |
If one of the first three fixes it, you've saved a call-out. If not, an engineer can diagnose a dead unit quickly — see air conditioning repair costs.
Start here
The fastest route from a dead unit to the cause.
What a fix costs (2026)
A diagnostic call-out runs £80–£150 and covers finding the fault plus minor work; parts are extra. The cheap fixes cost nothing but your time.
| Cause | Typical fix | Typical cost |
|---|---|---|
| Flat remote batteries or faulty handset | New batteries, or a replacement remote | £0 · ~£15–£30 remote |
| Isolator off or a one-off tripped breaker | Switch back on / reset once | £0 DIY |
| Repeated tripping or an electrical fault | Engineer traces the fault — call-out | £80–£150 + repair |
| Failed capacitor or contactor (outdoor) | Replace on the day — call-out plus part | £80–£150 + part |
| Indoor or outdoor control board (PCB) | Replace the board and test | £150–£400+ |
| Very old or obsolete unit | Replace rather than repair | £1,500–£3,000 new split |
Baselines: call-out £80–£150; single split installed £1,500–£3,000. Board and part prices vary by brand and model — get quotes. Full breakdown in air conditioning repair costs, or if the room is unbearable now, emergency repairs.
Keeping a dead unit from happening again
Two of the most common no-power causes are avoidable. A yearly service catches a tired capacitor or a corroding terminal before it strands you on the first hot day, and it keeps the manufacturer's warranty valid — most brands require documented servicing to honour a board or compressor claim. The other is surge protection: a lot of PCB failures trace back to a storm or a spike on the supply, and a plug-in or consumer-unit surge protector is cheap insurance for the electronics. If your unit still cools but has started behaving oddly — tripping now and then, or throwing the odd code — don't wait for it to die completely; a page like not cooling or the repairs hub will point you at the right next step. And if you're weighing up whether an ageing system is worth keeping alive at all, get it looked at before summer, not during it, when every engineer is booked solid.
Sources & methodology
- Diagnostic order reflects the no-power fault-finding sequence in manufacturer service manuals (Daikin, Mitsubishi Electric, Fujitsu, Panasonic, LG, Samsung).
- Electrical safety: work inside indoor or outdoor units involves mains voltage and stored charge and should be left to a competent, qualified engineer.
- Refrigerant handling: GB F-Gas Regulation (retained Regulation (EU) 517/2014) — only certified persons may recover, charge or check refrigerant; venting is an offence.
- Price baselines are 2026 UK figures used across this site: call-out £80–£150, single split installed £1,500–£3,000.
FAQs
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