Air conditioning not cooling, dripping or stopped: checks

Why there is no code on the wall

A domestic split has nothing to show a code on. The fault is recorded in the indoor unit's memory and comes out through the remote controller or as a pattern of flashing lights, and the procedure is different for every manufacturer. That is why so many people conclude their air conditioner has stopped for no reason: it has a reason, and it will tell you if you ask in the right way. Asking in the right way is the single most useful thing you can do before anyone attends, because the code decides what they bring.

• Daikin put the table in the owner's manual and publish the retrieval. Hold Cancel on the remote for five seconds until a blinking 00 appears in the temperature display. Press Cancel repeatedly and listen: a short beep, or two beeps, means the code showing is not yours; a long continuous beep means it is. Write that one down. Hold Cancel five seconds again to exit, or leave it a minute. Daikin group the table into SYSTEM, INDOOR UNIT and OUTDOOR UNIT, which tells you where the fault is before you have looked it up. 00 on its own means no fault is stored.
• Toshiba publish an instruction that inverts the universal first move: do not switch the supply off before reading the codes, because doing so clears the diagnostic memory. Press CHK on the wireless controller to enter service mode, step through with the TIMER buttons — the temperature buttons on some newer controllers, so follow the wording in your own manual — until all the lights flash with the buzzer, and note both halves — an initial code of 01, 02 or 03 and a detail code. Both are needed. Press CLR to clear, at which point the controller shows 7F, which is not a fault, and ON/OFF to leave. The green light flashing once a second at power-up is not a fault either.
• Mitsubishi Electric run a public lookup on their UK site with the cause and check procedure for each code. Two-character codes belong to the domestic and light commercial splits; four-digit codes to M-NET systems. One caution: the lookup covers Mitsubishi's heat pumps as well as its air conditioners, and the check procedure it returns is product-specific even where the headline fault is the same. U1 is a high-pressure fault on both a Mr Slim and an Ecodan, but the checks Mitsubishi list for each are different, so select the right product family before you read the steps.
• Other makes report the same fault several ways — a code, a lamp flash pattern, a wireless indicator and an app message — and searching the wrong representation finds nothing. Write the code exactly as shown, including a leading zero and whether a character is a letter or a digit. The retrieval page has the procedures in full.

Water dripping from the indoor unit

The indoor unit cools air below its dew point, and the water that condenses runs into a tray and out through a pipe to the outside. Mitsubishi name a blocked condensate pipe as the most common cause of water on the floor, and a dirty tray as the second: dust from the room and condensation make a sludge that blocks the tray outlet before it blocks the pipe. Turn the system off at the remote so nothing is adding water. Then go outside and find where the pipe comes through the wall, usually behind or below the indoor unit, and check the end is clear of leaves, algae and insect nests — it is the one part of the drain path an owner can inspect. Look at whether the pipe runs downhill all the way; a sagged or knocked section holds water and stops the rest draining. Take the indoor filter out. A blocked filter ices the coil, and when the unit stops the ice melts all at once and overwhelms the tray, so the symptom is water and the cause is airflow. If the unit has reported a code, retrieve it: a drain fault and a drain sensor fault look identical from the floor and lead to different repairs, and Mitsubishi publish a separate code for a sensor open or short with a perfectly clear tray. Do not open the indoor unit beyond the filter cover. The tray sits under the coil alongside the fan motor and mains wiring, and water and electricity are already in contact in there.

Not blowing cold

Most of these are free. Confirm the mode and setpoint on the remote first: fan mode moves air without cooling it, and a reversible unit left in heating does the opposite of what you want — both present as a machine running normally and achieving nothing. Then open the indoor unit's front flap and take the filters out; they are designed to come out without tools and this is explicitly an owner task. A filter that looks grey rather than white has been restricting airflow for a long time. Wash them, dry them completely, and while they are out look at the coil behind them for ice; if it is frozen, run the system in fan mode to thaw it before judging anything else. Daikin's A5 is published in the owner's manual as freeze-up or high-pressure protection; the cause list that names restricted indoor airflow is in Daikin's service documentation rather than the owner's manual, but a filter is still the one thing an owner can check before ringing. Then go outside. An outdoor unit that cannot reject heat — something too close to it, grown over it, or packed into the fins — cannot cool the room; clear the space around it. If the outdoor unit is not running at all, check the small weatherproof isolator on the wall beside it, which is switched off during other work more often than you would expect, check the breaker, and wait ten minutes: a compressor that has just stopped will not restart immediately, by design. In heating mode a unit that is steaming with the fan stopped is defrosting and will finish on its own.

If the filter is clean, the mode is right, the outdoor unit is clear and running, and the air is still not cold, the remaining causes are a loss of refrigerant, a compressor or a control fault, and none of them is yours. Daikin's U0 is a refrigerant shortage; E6 a compressor start failure; Toshiba's 03 codes are the outdoor side. A leak trace and recharge is substantial, and a compressor is the point at which replacing the system gets discussed.

The reset, and where the line is

The only reset worth attempting is switching off at the isolator or breaker, leaving it a minute, and switching back on — and on a Toshiba that is actively the wrong first move until the code has been read. Mitsubishi publish an owner reset for exactly one code, a non-defined error caused by noise interference, and it is thirty seconds off. For everything else the check procedures involve DIP switches inside electrical enclosures, service menus behind a password and gauges on the refrigerant circuit. Daikin publish an LED table for multi-split outdoor units, but reading it means removing a cover on a live outdoor unit, so it is not an owner diagnostic whatever it looks like online. Toshiba warn separately that high voltages are present if access covers are removed.

Handling the refrigerant requires F-Gas certification under the retained F-Gas Regulation — a legal gate rather than a competence norm, and the reason "call someone" here means someone specific. A large share of what these systems report has nothing to do with it: a tripped breaker, a blocked outdoor unit, a frozen condensate, a dirty filter. Do those. Then, if a code returns after one power cycle, treat it as a diagnosis and not a retry: a communication fault between the units (Daikin U4, Toshiba 01-04) is often an isolator, a breaker or a disturbed cable; a fan motor (Toshiba 01-11) or a drain component is an engineer's job; anything on the refrigerant side is F-Gas certified work.

Right now

If water is escaping indoors, isolate the system at its own switch or breaker rather than waiting for a call. Do not try to release pressure, top anything up or open the refrigerant pipework. If you can smell burning or see scorching at either unit, switch the circuit off at the consumer unit. In a heatwave, treat the heat as the risk rather than the appliance: a room that cannot be cooled matters most where someone elderly, very young or unwell is sleeping in it.

What it costs

The brackets are those the code pages carry. A drain clear and service, the usual outcome for water on the floor, £80 to £250, rising to £280 for a float switch or drain sensor and more for a pump, which Mitsubishi say is uncommon. A communication fault is nothing if it was the isolator or a breaker, and £380 to £400 at the top if a board at either end needs replacing. A fan motor with labour £150 to £450. A leak trace and recharge from about £200, and a coil or pipework re-run considerably more; a compressor runs to £900 to £1,200, which on an older split is frequently more than the system is worth. Reading the code costs nothing and is what stops you paying for a diagnosis you could have handed over.

Sources

• Daikin operation manuals for domestic splits (the fault code table by group, the Cancel-button retrieval, 00, A5, U0, U4, E6) and Daikin's multi-split outdoor unit LED table; the A5 cause list naming restricted airflow is from Daikin service documentation.
• Toshiba owner's and service documentation for domestic splits: the instruction not to switch off before reading codes, CHK service mode, initial and detail codes, 7F, the high-voltage warning. The stepping buttons are named TIMER in the documents used here and differ on some newer controllers.
• Mitsubishi Electric UK error code lookup (les.mitsubishielectric.co.uk): PA, 2503, U1, EF, the two code schemes and the noise-interference reset; Mitsubishi's drain-fault causes. The lookup returns U1 as a high-pressure fault for both Mr Slim and Ecodan, with product-specific checks.
• The retained F-Gas Regulation (Regulation (EU) 517/2014 as it applies in Great Britain) on certification for work on refrigerant circuits, as stated in this site's trade wording for the category.
• Cost brackets are those on the code pages named, each with its own sourcing.