Air conditioning error codes: understanding what your unit is trying to tell you before calling a technician
L'équipe Proclimo
9 Oct 2026 - 08 min read
There are two ways an air conditioner can fail. The first is silent: nothing lights up, the remote doesn't respond, and you go hunting for the breaker. The second is far more common and far more poorly understood: the unit shuts itself down, shows three characters on the indoor unit or blinks an LED in a specific rhythm, then locks itself out in safety mode.
That code isn't a gimmick. It's the result of continuous self-diagnosis: the control board constantly monitors a dozen or so temperature sensors, circuit pressure, fan speeds and the current drawn by the compressor. When a value falls outside the permitted range, it cuts power and stores the cause. Knowing how to read that message changes two very concrete things: you stop phoning about a clogged filter, and when you genuinely do call, you hand the technician the one piece of information that saves him half an hour — time that's usually billed.
Where to read the code, depending on the type of unit
Manufacturers don't all display codes the same way, and that's the first source of confusion.
Splits with a 7-segment display. This is the simplest case: two to four characters appear directly on the indoor unit (often behind the flap, on the right). Common on Mitsubishi Heavy, Toshiba, LG, Hisense and Haier.
Splits with no display, using blinking LEDs. Daikin, Mitsubishi Electric and Panasonic used this approach for years. The "timer", "operation" or "filter" LED blinks a certain number of times, pauses, then starts again. You have to count the blinks: 5 short blinks followed by a long pause doesn't mean the same thing as 2 long and 3 short. On Daikin units, you can also force the code to display by repeatedly pressing the timer cancel button on the remote until you hear a long beep — the remote then shows the stored code.
Ducted systems and multi-splits. The code appears on the wall-mounted wired controller, and it's generally the richest in information: some manufacturers also display the number of the indoor unit concerned (handy when four rooms are connected to a single outdoor unit).
Connected systems. If your installation has a Wi-Fi module, the manufacturer's app logs a time-stamped fault history. That's valuable: a code that reappears every day at 3 p.m. in the heat tells a very different story from a one-off code that occurred during a stormy night.
Worth remembering: note the code before cutting the power. Switching off at the breaker often clears the current fault, and you lose the only usable clue you had.
The shared logic behind manufacturer codes
The naming conventions differ, but the structure is the same everywhere. Around 90% of codes can be grouped into five families.
| Fault family | What the unit detects | Typical codes |
|---|---|---|
| Communication | The indoor and outdoor units no longer talk to each other | U4, UF, E6, EC, F1 |
| Temperature sensor | A thermistor is open-circuit or short-circuited | A5, E1/E2, F1/F2, P4 |
| Pressure / charge | High pressure, low pressure, refrigerant shortage | H6, E5, P1, P2, F3 |
| Fan motor | Fan blocked, power board faulty | H6, P5, E8 |
| Compressor overcurrent | The compressor is drawing abnormal current | U2, L5, P6, E4 |
In other words: the code doesn't tell you the faulty part, it tells you the suspected circuit. A Daikin U4 means "no more dialogue between the two units". That could be a burnt-out control board… or an interconnecting cable chewed by a rodent, a loose terminal block, or even a simple power fault on one phase. The same code covers a €20 repair and a €900 repair.
The codes homeowners encounter most often
- Communication fault (U4, E6, E1 depending on brand). Top of the call-out list. Classic causes: interconnecting cable damaged during façade work, poorly tightened terminal block (due to expansion and contraction cycles), moisture in the outdoor connection box, indoor board destroyed after a storm.
- Defrost sensor or heat exchanger sensor fault. Often after several winters: the thermistor comes away from its sleeve or its wire fatigues at the bending point. Cheap part, short labour time.
- High pressure fault. In cooling mode in summer, the leading cause isn't a breakdown but insufficient heat exchange at the outdoor unit: grille blocked by leaves, caked pollen, garden furniture pushed up against the unit, decorative housing with inadequate ventilation. In heating mode in winter, it's more likely a clogged indoor filter or a slowed indoor fan.
- Indoor fan fault. Typical when the blower wheel has never been cleaned: the build-up increases torque, the motor can no longer hold the requested speed, and the board shuts down.
- Anti-freeze protection or icing fault. The unit goes into safety mode because the evaporator is dropping too low. Dirty filters, insufficient airflow, or an incorrect refrigerant charge.
The six legitimate checks to make before calling
Nothing that follows touches the refrigerant circuit. These are checks any homeowner can carry out, and they resolve a far from negligible share of "breakdowns".
1. The indoor unit filters. We keep hammering this point because it is still, always, the number one cause. Lift the flap, pull out the filters, hold them up to the light. If they look grey, they're already restricting airflow. A wash in lukewarm water, thorough drying in the shade, and back they go. Recent models also accept replacement activated carbon filters which must be replaced rather than washed — check the manual before you get anything wet.
2. The surroundings of the outdoor unit. Clear at least 30 cm in front of the discharge grille and 10 cm at the rear. Remove leaves, maple seeds, wasp nests, stored cardboard. In suburban gardens, grass clippings blown against the coil are a summer classic.
3. The outdoor coil itself. Look between the fins: if light no longer passes through, heat exchange is compromised. Gently working a soft brush along the direction of the fins, with the fan stopped and the unit powered down, is often enough. Absolutely avoid a pressure washer, which flattens the fins and permanently degrades performance.
4. Condensate drainage. Many units shut down when a high level is detected in the tray. If there's a float switch, a blocked drain will trigger a code. Check that the outdoor drain pipe flows freely.
5. The electrical supply. Check the dedicated breaker, but also its condition: a breaker that has tripped several times can stay "tired". If the panel has a surge protector, check its indicator. If the code relates to communication, a full reset (power off for 5 minutes, then back on) will sometimes clear a transient fault — though it never provides a lasting fix if the fault returns.
6. The remote control settings. Before talking about a breakdown: "fan only" mode switched on by mistake, setpoint left at 28 °C in midsummer, "quiet eco" mode throttling the outdoor unit, a forgotten schedule. And above all, the batteries: a weak remote sends incomplete signals and the unit behaves erratically. Keeping a pack of AAA alkaline batteries in the drawer avoids a false alarm.
If the code comes back after these six points, the problem is real and you need a professional.
What you can't do, and it isn't a question of skill
The boundary is legal, not technical. Any work on the refrigerant circuit — connecting a manifold, checking a pressure, adding or recovering refrigerant, brazing a line — is restricted to operators holding a certificate of competence working for a company holding a company certification, in line with the European framework on fluorinated gases (F-Gas Regulation) as transposed into French law. Selling, buying or handling R-32 or R-410A without those credentials is illegal, and the "AC recharge" cans sold online for cars have absolutely no business anywhere near a domestic split system.
Two practical consequences that are often overlooked:
- The warranty. Any sign that the circuit has been opened by a non-professional voids the manufacturer's warranty, including on electronic parts.
- Insurance. In the event of damage resulting from non-compliant work, your home insurer may refuse to cover the claim. The issue also ties in with the ten-year structural warranty on the original installation.
The right instinct: document, don't dismantle. A photo of the code, a photo of the outdoor unit's rating plate (exact model, refrigerant type, charge in kg), and a description of the context are worth far more than an opened cover.
Preparing the call so you don't pay for two visits
A air-to-air heat pump repair call-out generally costs between €90 and €180 for the visit and first hour, excluding parts, with wide variation by region and season. The worst-case scenario is the double visit: the technician comes, diagnoses, leaves to order the part, then comes back. You can reduce that risk.
On the phone, give the following, in order:
- The exact brand and model reference of both units (indoor and outdoor), taken from the rating plate — not the sales brochure.
- The code displayed, exactly as shown, and how you read it (display or number of blinks).
- The operating mode when the fault occurred: cooling, heating, defrost, start-up.
- How often it happens: once, at every start-up, after X minutes, only when it's very hot.
- What has changed recently: building work, a storm, a power cut, furniture moved, a housing built around the outdoor unit.
- The date of the last service and who carried it out.
These six items often allow the technician to arrive with the likely sensor or board already in the van. To keep track of all this over time, a heating and air conditioning service logbook tucked near the electrical panel does a job that neither memory nor email does properly: you record the codes you've seen, the dates, and the parts replaced.
Three ways to anticipate codes rather than suffer them
Keep an eye on consumption. An emerging fault often shows up on the bill first. A compressor working against a fouled coil draws more power for the same comfort level for weeks before it finally shuts down. A plug-in smart wattmeter obviously can't be fitted to an air conditioner's dedicated circuit, but on a single-phase installation equipped with a sub-meter or a clamp meter placed on the supply cable, the drift becomes visible within a few days.
Measure the temperature differential. In cooling mode, the difference between the air drawn in at the top of the indoor unit and the air blown out should typically sit between 8 and 12 °C in steady-state operation. Below 6 °C, something is wrong: filter, airflow or charge. An infrared non-contact thermometer makes this check instant and gives you a numerical history to pass on to the technician.
Don't confuse servicing with repairs. The periodic leak check required by F-Gas regulations applies to equipment containing 5 tonnes of CO₂ equivalent or more of refrigerant — which leaves out the majority of domestic splits. But the absence of a legal obligation doesn't mean the absence of a need: annual cleaning of the heat exchangers and a visual check of the connections remain the best insurance against error codes. In between, an evaporator cleaning spray used according to the instructions is a useful complement to washing the filters, though it never replaces professional servicing.
In summary
An error code isn't a verdict, it's a pointer. It identifies a circuit, rarely a part. Communication, sensor and pressure faults account for the bulk of what refrigeration technicians see in private homes, and a significant share of them stem from a dirty filter, a blocked coil or a loose terminal block — in other words, avoidable causes.
The right approach comes down to three steps: read and note the code before cutting any power, carry out the six non-refrigerant checks, then call with a complete file if the fault persists. Beyond that, the line is clear: as soon as the refrigerant circuit is involved, only a company holding the appropriate certification can legally intervene. That boundary isn't pointless red tape: it protects the environment, your warranty and your insurance cover all at once.
Regulatory sources: European regulation on fluorinated greenhouse gases (F-Gas) and its transposition into French law (Code de l'environnement, articles R. 543-75 onwards); ANSES recommendations on indoor air quality and the maintenance of air treatment systems; manufacturer manuals from Daikin, Mitsubishi Electric, Toshiba, LG and Atlantic for code listings.
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