Air Conditioning Not Cooling: Fault-Finding Guide

Summary

"The air con isn't cooling" is one of the vaguest fault descriptions a tradesperson hears, covering everything from a five-minute filter clean to a compressor failure needing a full replacement. A systematic decision tree stops engineers (and homeowners on the phone) jumping straight to "it needs re-gassing" — the most expensive assumption, and usually the wrong one. A genuine refrigerant leak is a real and common cause of poor cooling, but it typically sits behind several cheaper, faster, legally unrestricted checks.

This matters to general AC and refrigeration engineers, electricians and plumbers who increasingly fit and service domestic split systems, and anyone fielding a call-out where the customer has already decided (usually wrongly) what's broken. It also matters because of the legal boundary running through the middle of this fault tree: anyone can check a filter, clear a condensate line, or investigate a thermostat setting, but only an F-Gas certified engineer may connect gauges to the refrigerant circuit, recover refrigerant, or add charge.

The most common misconception is that "not cooling" always means "needs refrigerant." In reality, airflow restriction (a dirty filter or fouled outdoor coil) and condensate-related safety lockouts account for a large share of domestic call-outs, resolved without ever touching the sealed circuit. Work the cheap checks first — it's faster for the customer and the only way to know whether refrigerant work is genuinely needed before booking a certified engineer's time.

Key Facts

• F-Gas Regulations — Fluorinated Greenhouse Gases Regulations (SI 2015/310, as amended, most recently by SI 2023/1161) — only certified technicians may handle, recover or recharge refrigerant; civil penalties up to £200,000 per offence for non-compliance.
• Certification routes — City & Guilds 2079 Parts 1 and 2 (or equivalent) with REFCOM or an equivalent F-Gas registration; Category I covers all equipment, Category II covers systems below 3kg charge (6kg hermetically sealed).
• Filter fouling — a blocked evaporator filter can cut rated cooling capacity by 15–25% within a single season of neglect; the single most common cause of "weak" (not zero) cooling.
• Filter maintenance interval — every 6–8 weeks in heavy use for standard washable mesh filters; this is a customer task, not an engineer call-out.
• Condenser (outdoor unit) fouling — leaves, cottonwood, insect nests and general dirt block the outdoor coil, restricting the heat rejection the system depends on to cool the building.
• Outdoor unit clearance — minimum roughly 300mm rear clearance and 1m front clearance is typical (check the specific manufacturer's installation manual); blocked airflow degrades performance and can trigger high-pressure trips.
• Condensate drainage faults — a blocked condensate tray, drain line or failed condensate pump is a very common cause of the system shutting down entirely (many units have a float switch that stops the compressor to prevent an internal water overflow) — this is frequently misread by the customer as "it's stopped cooling" when it has, correctly, stopped itself.
• Refrigerant pressure checks — suction (low side) and discharge (high side) pressures must be compared against manufacturer data for the prevailing ambient conditions; only a certified engineer may connect gauges to a sealed system.
• Superheat and subcooling — the two derived measurements (from pressure and temperature readings) that confirm whether the refrigerant charge and expansion device are working correctly; interpreted against the manufacturer's target values, not a generic rule of thumb.
• Refrigerant weigh-in — any refrigerant added must be weighed on calibrated scales and logged; estimating a top-up "by feel" is not compliant practice.
• Common refrigerants — R410A (GWP 2088, non-flammable A1, phased out of new equipment from 2025) and R32 (GWP 675, mildly flammable A2L, now dominant in new split systems) behave differently under fault conditions and require different handling precautions.
• Temperature differential test — the difference between supply (cooled) and return air at the indoor unit should typically be around 10–14°C in cooling mode under normal conditions; a smaller differential points to an airflow, refrigerant or expansion valve problem.
• Thermostat/controls faults — incorrect mode selection, a stuck or miscalibrated room sensor, a dead controller battery, or a scheduling conflict on smart/zoned systems are all common causes that present exactly like a "not cooling" refrigerant fault to the customer.
• Compressor short-cycling or failure to start — often an electrical fault (capacitor, contactor, overload trip) rather than refrigerant-related; electrical isolation and safe working under the Electricity at Work Regulations 1989 applies before any panel is opened.
• BS EN 378 — the British/European standard covering safety and environmental requirements for refrigerating systems, including classification of refrigerants by toxicity and flammability.

Quick Reference Table

Detailed Guidance

Fault-Finding Decision Tree

START: Customer reports "AC not cooling"
│
├─ STEP 1: Is the system running at all? (indoor unit fan, outdoor unit fan/compressor)
│   ├─ NO — nothing runs
│   │   ├─ Check power supply / isolator / RCD / breaker tripped
│   │   │   ├─ Tripped repeatedly → electrical fault (compressor, capacitor, wiring) — call AC/electrical engineer
│   │   │   └─ Reset holds → monitor; check thermostat call for cooling
│   │   └─ Check thermostat/controller: correct mode (cooling, not fan/heat), setpoint below room temp, schedule not blocking
│   │       ├─ Controller fault / dead batteries / no display → replace/reset controller
│   │       └─ Controller fine, system still won't start → call AC engineer (control board or contactor fault)
│   │
│   └─ YES — something is running
│       ├─ STEP 2: Is the INDOOR fan blowing air?
│       │   ├─ NO airflow at all → check filter fitted correctly, fan speed setting, blower fault → any engineer
│       │   └─ YES, air is moving → proceed to Step 3
│       │
│       ├─ STEP 3: Is the air coming out WARM or only slightly cool (not properly cold)?
│       │   ├─ Check filter — is it visibly blocked/dirty?
│       │   │   ├─ YES → clean/replace filter, re-test after 15–20 min
│       │   │   └─ NO → proceed
│       │   ├─ Check OUTDOOR unit — is the coil clean and clear (no leaves/debris/ice)?
│       │   │   ├─ Blocked/dirty coil → clean coil, check clearances, re-test
│       │   │   ├─ Coil iced up → is it defrosting normally, or staying iced?
│       │   │   │   ├─ Melts within 15–20 min after stopping → likely airflow-related, monitor
│       │   │   │   └─ Stays iced / re-ices quickly → possible low refrigerant charge or expansion valve fault → F-Gas certified engineer
│       │   │   └─ Coil clean, no ice, fan running normally → proceed
│       │   └─ Check temperature differential at indoor unit (supply vs return)
│       │       ├─ Differential well below ~10°C, all airflow checks clear → refrigerant-side fault suspected → F-Gas certified engineer to check pressures, superheat/subcooling
│       │       └─ Differential normal but room still doesn't reach setpoint → check system sizing, room heat gains, or thermostat sensor placement
│       │
│       └─ STEP 4: Has the system stopped ON ITS OWN mid-cycle, or shows a fault code?
│           ├─ Water visible near indoor unit / drain pan → check condensate drain and pump
│           │   ├─ Blocked drain/tray → clear and flush, check fall, re-test
│           │   └─ Condensate pump failed (float switch/non-return valve) → replace/repair pump
│           ├─ Fault code on display → look up code in manufacturer's manual
│           │   ├─ Code indicates sensor/electrical fault → qualified engineer, no refrigerant access needed
│           │   └─ Code indicates pressure/refrigerant fault → F-Gas certified engineer required
│           └─ No code, just stopped → check for overheat/high-pressure protection trip (blocked condenser, high ambient) → clear obstruction, allow to cool, reset
│
└─ If all airflow, controls and condensate checks are clear and performance is still poor:
    → Escalate to an F-Gas certified engineer for pressure, superheat/subcooling and leak-detection checks.
      Do not attempt to connect gauges, recover, or add refrigerant without F-Gas certification —
      this is a legal requirement, not best practice.

Airflow restriction — the most common cause

Start every "not cooling" call-out here — it accounts for a large share of genuine faults and needs no refrigerant access. A blocked evaporator (indoor) filter reduces airflow across the coil and can cut rated cooling capacity by 15–25% within a single season if neglected — the system runs, blows air, but that air is only mildly cool. Check and clean or replace the filter first, always, before assuming anything more serious.

Equally common and equally overlooked: the outdoor condenser coil, where the system actually rejects the heat pulled from inside. If it's clogged with leaves, cottonwood, insect debris or general dirt, the whole system struggles regardless of how clean the indoor filter is. Check the manufacturer's minimum clearances (commonly around 300mm rear, 1m front — confirm against the specific installation manual) haven't been compromised by fencing or planting, and clean the coil fins with a coil cleaner or a fin comb if bent.

Condensate drainage — the "stopped cooling" that isn't a cooling fault at all

A large proportion of "the AC just stopped" call-outs aren't cooling faults at all — they're the system protecting itself from an internal water overflow. Most split systems have a float switch in the condensate tray or pump that shuts the compressor down if condensate can't drain away, whether the tray or drain line is blocked with biofilm and dust, or a condensate pump has failed (stuck float switch, failed non-return valve, or a burned-out pump). This presents exactly like a refrigerant or compressor fault — "it was cooling fine, then it just stopped" — but is resolved by clearing and flushing the drain path or replacing the pump, with no refrigerant work involved.

Thermostat and controls faults

Before assuming a mechanical or refrigerant fault, rule out the controller. Check the system is actually in cooling mode (not fan-only or heating on a reversible unit), the setpoint is genuinely below room temperature, no active schedule or "away" mode is overriding the manual setting, and battery-powered wall controllers aren't flat. On smart or zoned multi-split systems, check that a master controller or zone damper isn't blocking flow to the room. These faults are common, quick to diagnose, and cost the customer nothing beyond the call-out.

Icing, low charge and when to call an F-Gas engineer

Ice forming on the indoor coil is a classic symptom with several possible causes. Restricted airflow (a blocked filter, closed vents, or a failing indoor fan) is the most common and cheapest to fix — clear the airflow path and the icing should resolve within a normal defrost cycle. Persistent or rapidly recurring icing, especially alongside falling cooling performance over weeks or months, points toward a genuine refrigerant undercharge from a slow leak, or an expansion valve fault. At this point the job crosses the legal line: only an F-Gas certified engineer may connect gauges to check suction and discharge pressures, calculate superheat and subcooling against manufacturer targets, leak-test the system, and — if confirmed — recover and recharge refrigerant, weighed on calibrated scales and logged in the F-Gas record. See f gas regulations guide for certification and record-keeping requirements, and refrigerant types comparison for how R32 and R410A behave differently in fault conditions.

Electrical faults

A compressor that hums but won't start, or trips the breaker on start-up, is very often an electrical fault — a failing run capacitor, a worn contactor, or an overload trip — rather than a refrigerant problem. This work requires safe isolation under the Electricity at Work Regulations 1989 before any panel is opened, and diagnosis (capacitor testing, contactor inspection, motor winding checks) sits with a qualified electrical or AC engineer. Don't default to "it needs re-gassing" just because the outdoor unit isn't running — confirm power reaches the compressor and the electrical components are sound before considering the refrigerant circuit at all.

When to escalate immediately

Some symptoms should go straight to an F-Gas certified engineer without further general-trade diagnosis: a hissing or bubbling noise at a pipe joint, visible oily residue around a fitting (oil circulates with refrigerant, so a residue is a classic leak sign), or a fault code that explicitly references pressure or refrigerant. These are refrigerant-circuit symptoms from the outset and don't benefit from filter or condensate checks first.

Frequently Asked Questions

Can I check refrigerant levels myself if I'm not F-Gas certified?

No. Connecting gauges to the refrigerant circuit, checking pressures, or adding or recovering refrigerant in any quantity is restricted to F-Gas certified technicians under the Fluorinated Greenhouse Gases Regulations. This applies regardless of system size or refrigerant type — R32 and R410A are both covered. A general engineer or electrician can rule out every non-refrigerant cause (filters, condenser fouling, condensate, controls, electrical faults) but must hand off to a certified engineer once refrigerant work is genuinely needed.

The system is icing up — does that always mean low refrigerant?

No. Icing is most commonly caused by restricted airflow — a blocked filter, closed or obstructed vents, or a failing indoor fan — which is a straightforward, unrestricted fix. Persistent or rapidly recurring icing that doesn't resolve once airflow is confirmed clear is the point at which a refrigerant undercharge or expansion valve fault becomes the likely cause, and that diagnosis (pressure and superheat/subcooling checks) requires an F-Gas certified engineer.

Why did the system just stop running with no warning?

Check the condensate drain and pump before assuming a mechanical failure. Many systems shut the compressor down automatically via a float switch if condensate can't drain away, to prevent an internal water overflow — this is the system working correctly, not a fault with the cooling components themselves. Clearing a blocked drain or fixing a failed condensate pump resolves a large share of these "sudden stop" call-outs with no refrigerant work involved.

How much cooling capacity can a dirty filter or condenser actually cost?

A blocked evaporator filter alone can cut rated cooling capacity by roughly 15–25% within a single season of neglect. A fouled outdoor condenser coil compounds this further, because the whole system depends on the outdoor coil to reject the heat pulled from inside — restricted airflow there degrades performance across the board, independent of the filter condition indoors.

Is it worth trying a reset before calling an engineer?

Yes, as a first step for a system that has stopped without obvious cause: check the thermostat mode and setpoint, confirm the isolator or breaker hasn't tripped, and power-cycle the unit per the manufacturer's instructions. If the fault clears and doesn't recur within 24 hours, it was likely transient. If it recurs, or a fault code reappears, book a proper diagnostic visit rather than repeatedly resetting a genuine fault.

Regulations & Standards

• Fluorinated Greenhouse Gases Regulations (SI 2015/310, as amended, most recently by SI 2023/1161) — governs who may handle, recover and recharge refrigerant, leak check obligations, and record-keeping.

• Electricity at Work Regulations 1989 — governs safe isolation and electrical fault-finding on the compressor, controls and wiring before any panel is opened.

• BS EN 378 — safety and environmental requirements for refrigerating systems, including refrigerant classification by toxicity and flammability.

• F-Gas certification standards — City & Guilds 2079 Parts 1 and 2 (or equivalent), assessed and registered via REFCOM or an equivalent UK F-Gas certification body.

• Approved Document F (Ventilation) — relevant where poor room ventilation or heat gain, rather than an AC fault, is contributing to inadequate cooling performance.

• Fluorinated Greenhouse Gases Regulations 2015 — legislation.gov.uk — the UK statutory instrument governing F-Gas handling.

• HSE — Electricity at Work Regulations 1989 — safe isolation and electrical fault-finding duties.

• REFCOM — F-Gas technician and company certification register.

• British Standards Institution (BSI) — BS EN 378 and related refrigeration safety standards.

• f gas regulations guide — full F-Gas certification, leak-check frequency and record-keeping requirements

• refrigerant types comparison — R32 vs R410A vs R290 behaviour, GWP and flammability classification

• ac maintenance annual service — the scheduled service checks that prevent most of these faults before they cause a call-out

• air conditioning installation pricing guide — pricing context for when a fault points toward repair vs replacement