Emergency Air Conditioning Repair in London

Who We Are?

J SONS & CO. repairs commercial and residential air conditioning systems across London and Harrow or all over the UK, from a single split that has stopped cooling to a multi-floor VRF system showing fault codes across a building. Same-day response in London. 24/7 emergency response in London through the summer. Engineers in marked vans carry common spares for every major brand, so most call-outs are resolved on the first visit.

Our London engineers are F-Gas certified, REFCOM Elite registered, and NICEIC approved. Every repair is documented with a written diagnostic report, refrigerant log entries where applicable, and a fixed-price quote for any parts before work proceeds. No surprises on the invoice.

Common AC Faults: Symptoms, Causes, and What to Do

Air conditioning faults rarely arrive without warning. Most failures show 2–5 days of symptoms before the system stops working entirely. Knowing the symptoms means catching faults early, when the fix is £80, not £800.

AC Running But Not Cooling

Most likely causes: low refrigerant charge (refrigerant leak somewhere in the system), blocked condenser coil, blocked indoor filter, frozen evaporator coil, failed compressor capacitor, faulty TXV/EEV expansion valve.

What you can check: indoor filter cleanliness, outdoor condenser cleanliness (clear of leaves, dust, debris), thermostat setpoint and mode. If the filters and condenser are clean and the unit still won’t cool, call us.

Why it matters: Running an AC system that is short on refrigerant burns the compressor out. Compressors are the most expensive component to replace. Switch the system off and call before the compressor fails.

AC Not Turning On At All

Most likely causes: tripped circuit breaker, failed isolator switch, blown control board fuse, failed contactor, dead remote (residential), thermostat fault, BMS communication fault (commercial).

What you can check: circuit breaker is on at the consumer unit/distribution board, indoor isolator switch is on, and remote batteries (residential).

When to call: if the breaker keeps tripping after reset, do not keep resetting it, there is a short or earth fault somewhere in the system that needs an engineer with electrical qualifications.

AC Leaking Water Indoors

Most likely causes: blocked condensate drain line, failed condensate pump, frozen evaporator coil (defrosting), drain pan corrosion, indoor unit incorrectly levelled at installation.

What you can check: turn the unit off at the remote (not the breaker) and let it drain. If water flow stops, the cause is condensate-related and needs an engineer.

Why it matters: ongoing leaks cause ceiling damage, mould growth and in commercial installations can trigger water-detection alarms. Same-day call-out recommended.

AC Making Loud Noises

Most likely causes by sound type:

• Grinding or screeching: fan motor bearing failure, fan blade strike, compressor failure.
• Clicking or buzzing: failing contactor, capacitor issue, control board relay problem.
• Hissing or whistling: refrigerant leak (often audible at flare joints), restricted airflow.
• Banging or knocking: compressor internal damage, loose mounting bolts on the condenser.

When to call: any new mechanical noise from an AC system. Mechanical noises rarely fix themselves; they progress to failure.

AC Blowing Warm Air

Most likely causes: refrigerant leak (most common in summer), failed compressor or compressor contactor, reversing valve fault (heat pumps stuck in heat mode), or outdoor unit not running.

What you can check: whether the outdoor unit is running. If the indoor is running but the outdoor is dead, the fault is at the outdoor end (capacitor, contactor, fan motor, compressor).

Why it matters: in London, in the summer, this is an emergency, call immediately. In Harrow or UK, it is uncomfortable but rarely dangerous; a same-day or next-day response is usually fine.

AC Smells Bad When Running

Most likely causes: bacterial or fungal growth on the indoor coil (“dirty sock syndrome”), blocked drain line with stagnant water, electrical insulation burning (urgent), dead rodent or pest in ductwork (commercial).

When to call: any burning or electrical smell, turn the system off at the breaker and call immediately; this is a fire risk. Musty or mildew smells can wait for a routine appointment with an antimicrobial coil clean.

AC Showing Fault Code on Display

Modern Daikin, Mitsubishi Electric, Toshiba, LG, Panasonic, Hitachi and Samsung units display fault codes on the indoor unit or wired remote. Each manufacturer uses different codes, quote the code when you call, and we will arrive with the likely required parts.

Common Daikin codes: U0 (low refrigerant), U4 (communication error indoor-outdoor), E5 (compressor overload), F3 (high discharge temperature). Common Mitsubishi codes: P5 (drain pump fault), P8 (pipe temperature error), E6 (indoor-outdoor communication error). Common LG codes: CH 05 (communication error), CH 21 (compressor IPM fault), CH 33 (refrigerant low pressure). Common Samsung codes: E101 (communication error), E461 (compressor start-up fail), E471 (outdoor PCB fault).

AC Short-Cycling (Switching Off and On Frequently)

Most likely causes: oversized AC system (installed wrong from day one), thermostat positioned in direct sun or near a heat source, low refrigerant charge, frozen evaporator triggering pressure cut-out, or capacitor failure on compressor start.

Why it matters: Short-cycling destroys compressors faster than any other operating condition. Every start draws 5–8× the running current. Call early.

Refrigerant Leak Detected

Symptoms: ice forming on the indoor coil or refrigerant pipework, hissing sounds at joints, oily residue at flare connections, and gradually reducing cooling capacity over days or weeks.

What we do: electronic leak detection (UV dye method for hard-to-find leaks), repair the leak, evacuate the system to deep vacuum, and recharge with the correct refrigerant by weight. Record entry in the F-Gas log (UK). We do not top up refrigerant without finding and fixing the leak; this is illegal in the UK and bad practice everywhere.

AC Repair in London

Emergency Response Times

J SONS & CO. London operates a tiered response structure across the M25:

Priority sectors: restaurants, dark kitchens, healthcare clinics, server rooms, data centres, hotels and care homes, get prioritised dispatch during business hours.

London Service Coverage

Our London engineers cover all 32 boroughs and the City of London. Active emergency repair areas include the City, Canary Wharf, Mayfair, Soho, Shoreditch, Holborn, King’s Cross, Camden, Westminster, Kensington, Chelsea, Hammersmith, Fulham, Battersea, Vauxhall, Wandsworth, Clapham, Greenwich, Stratford, Wembley, Croydon, Bromley and Romford. M25 corridor projects covered on request.

F-Gas Compliance on Every Repair

Every refrigerant-related repair in the UK must be carried out by an F-Gas certified engineer, recorded in the system F-Gas log, and the refrigerant must be recovered, recycled or destroyed properly. J SONS & CO. holds the F-Gas company certificate, and every refrigerant engineer holds an individual F-Gas qualification. Using an uncertified contractor for a refrigerant repair is illegal, uninsurable and a regulatory risk under F-Gas Regulations 2015 (civil penalties up to £200,000).

London AC Repair Pricing

Indicative pricing. The final price is quoted before any chargeable work starts on site. VAT additional.

Repair vs Replace: Honest Assessment

Three rules we apply when advising whether to repair or replace:

1. If repair cost exceeds 50% of replacement cost AND the system is over 10 years old, replacement is usually the better commercial decision. We tell you that, even though replacement is a larger job for us than a repair.
2. If the refrigerant is R22 (banned for top-up) or R407C (being phased down), replacement should be planned within 2–3 years. Continuing to repair these systems is a short-term measure only.
3. If the compressor has failed and the system is over 8 years old, replacement is almost always more cost-effective than a compressor swap.

We will tell you the honest answer at diagnosis, not the answer that gives us the bigger invoice.

AC Brands We Repair

We diagnose, source parts for, and repair every major commercial and residential AC brand sold in the London or all over the UK Daikin, Mitsubishi Electric, Toshiba, LG, Panasonic, Samsung, Hitachi, Fujitsu, Trane, Carrier, York, GREE, Midea, SKM, Mitsubishi Heavy Industries. Our vans carry common spares for the top six brands, so most repairs are completed on the first visit. For less common brands or specialist VRF systems, we may need to source parts, same-day where local stock allows, 2–7 days for parts shipped from manufacturer warehouses.

Sectors We Repair AC For

Offices, co-working spaces, corporate HQs, retail stores, supermarkets, restaurants, cafés, dark kitchens, hotels, boutique hotels, schools, nurseries, universities, healthcare clinics, dental practices, GP surgeries, veterinary surgeries, private hospitals, gyms, leisure centres, hair salons, beauty clinics, warehouses, distribution centres, data rooms, server rooms, places of worship, listed buildings, embassies, residential apartments, villas and luxury private homes.

Why Clients Choose J SONS & CO. for AC Repair

Diagnosis on the phone where possible. Quote the fault code, describe the symptoms, and our engineers will often identify the likely cause before the van leaves. That means the right parts arrive on the first visit.

Spares in the van. Our vans carry stocked common spares, capacitors, contactors, fan motors, condensate pumps, drain pumps, common PCBs, for Daikin, Mitsubishi Electric, Toshiba, LG, Panasonic and Hitachi. First-visit fix rate is typically 70–80% for in-stock repairs.

Fixed-price quote before chargeable parts work. The diagnostic fee is the only fee until you approve the quote for parts work.

F-Gas in London, Refrigerant handling is fully compliant in both jurisdictions. Service log entries are made on every visit.

Honest repair-vs-replace advice. If repair doesn’t make commercial sense, we say so, even though replacement is a bigger job for us.

12-month workmanship warranty on every repair. Replacement parts carry their own manufacturer’s warranty in addition.

Frequently Asked Questions

How quickly can you respond to an AC breakdown?

In London, the target response is 2 hours in Central London during office hours, same-day across Greater London. Priority sectors (hospitality, healthcare, server rooms, hotels) get faster dispatch.

What does an AC repair cost?

In London, a typical call-out plus diagnostic is £125–£185 for commercial, £95–£145 for domestic. Parts and additional labour are quoted before work proceeds. 

Do you fix AC systems you didn’t install?

Yes. The majority of our repair work is on systems originally installed by other contractors. We work on every major brand.

My AC is making a loud noise. Is it dangerous?

It depends on the noise. Grinding, screeching, banging or knocking sounds usually indicate mechanical damage progressing to failure; call same-day. Hissing or whistling can indicate a refrigerant leak; call same-day in summer, next-day in winter. Buzzing or clicking can indicate electrical issues; call to be safe.

There’s a burning smell from my AC. What should I do?

Turn the system off at the circuit breaker immediately and call for emergency response. Burning smells indicate electrical insulation failure, which is a fire risk. Do not continue running the system. This is one of the very few AC faults that genuinely cannot wait.

My AC keeps tripping the circuit breaker. Why?

There is either an electrical short, an earth fault, or a component drawing excessive current. Do not keep resetting the breaker; repeated trips can damage the breaker, the AC components, and create a fire risk. Call for diagnosis the same day.

Should I top up the refrigerant myself?

No. In the UK, it is illegal under F-Gas Regulations for anyone without an F-Gas qualification to handle refrigerant. Adding refrigerant without finding the leak first is also pointless; the gas will leak out again within weeks or months.

Is it worth repairing an old AC system, or should I replace it?

Three rules: if the repair cost exceeds 50% of the replacement cost and the system is over 10 years old, replace. If the refrigerant is R22 (banned for top-up) or R407C, plan replacement within 2–3 years. If the compressor has failed and the system is over 8 years old, replace. We give honest repair vs replace advice on diagnosis.

Does my landlord need to approve an AC repair?

In London, repair work on a leasehold flat usually does not need freeholder approval, only installation does. 

Call for Emergency AC Repair

For London emergencies, phone: +44 20 8050 7969

Emergency (contract clients): 24/7

Email: info@jsonsco.com

Address: Wembley, London, United Kingdom, HA9 

Or use our contact form for non-emergency repair enquiries a member of the technical team will respond within one working hour during office hours.