Common Air Conditioner Error Codes Explained

You've walked past the indoor unit and noticed the display isn't showing the temperature anymore — instead there's a code like "U4" or "P6", or the operation light is just flashing on its own. It's a common moment of panic for Adelaide homeowners, especially mid-summer when the last thing anyone wants is a dead air conditioner. The good news is that the code is the unit telling you something specific. The less convenient news is that what it's telling you depends entirely on which brand and model you own.

At Voltas Electrical & Airconditioning we service and repair all 20 major brands sold in Adelaide — Daikin, Mitsubishi Electric, Mitsubishi Heavy Industries, Panasonic, Fujitsu General, LG, Samsung, Toshiba, Hitachi, Haier, Gree, Midea, Carrier, York, ActronAir, Temperzone, Braemar, Rinnai, Kelvinator and TCL — under licences BLD 274739, PGE 265145 and AU 61210. Our technicians read fault codes against the actual manufacturer service documentation for that model every day, which is exactly why we're not going to hand you a single "master list" of codes below. Instead, here's how the codes are actually structured, some real, verifiable examples from major brands, and — more importantly — how to tell when a code means "try this yourself" versus "call someone now."

Why there's no universal error code table

Every air conditioner manufacturer designs its own control board logic, so the same two-character code can mean entirely different things between brands, and sometimes even between different model ranges from the same brand. A "U4" on one system and a "U4" on another aren't guaranteed to be the same fault. Any website or generator that hands you a single all-brands code table without asking your model number is guessing — the only reliable way to read a code is against that specific manufacturer's service documentation, or a technician who has access to it.

Display style also varies. Split systems with a wired wall controller usually show an alphanumeric code directly (e.g. "E:04" or "P6"). Many wall-mounted units with only a handheld remote instead flash a pattern on the indoor unit's operation and timer LEDs — a certain number of red flashes paired with a certain number of green flashes maps to a specific fault in that model's service manual. If your unit does this, it's worth filming the flash pattern on your phone before calling anyone, since it's easy to miscount in the moment.

Real examples from major brands (not a generic table)

These are genuine, currently documented examples pulled from manufacturer-referenced service information for brands Voltas commonly services — treat them as illustrations of how each brand's system works, not as a substitute for checking your own unit's exact code against its model documentation.

Sources: Daikin service documentation summarised via star-supply.com; Mitsubishi Electric's own Mr Slim error code reference (mitsubishielectric.com.sg); Panasonic fault code references summarised via idealairconditioningservice.com.au. If your unit shows any of these exact codes, still confirm against your own model's documentation or ask your technician — model variants can differ.

The four broad fault categories behind most codes

Even without a universal table, almost every brand's codes sort into the same handful of underlying fault categories. Knowing which category you're in tells you roughly how serious the problem is, long before a technician arrives.

Communication errors

These indicate the indoor and outdoor unit (or the indoor unit and its wired controller) have stopped exchanging data properly. Causes range from a loose terminal or a nicked cable to a failed communication module on the control board. It's rarely dangerous, but it does need a technician to trace the wiring run and test the boards — the fault won't fix itself with repeated resets.

Sensor faults

Thermistors and pressure sensors feed the control board the data it uses to run the system safely. A sensor fault code means the board is either getting no reading, an implausible reading, or a reading outside expected range from one of these sensors. Sometimes this is the sensor itself failing; other times it's a symptom of a bigger issue (like low refrigerant) confusing what should be a normal reading.

Drainage and float switch faults

Every split and ducted system relies on condensate draining away from the indoor unit. A blocked drain line, a jammed float switch, or a failed drain pump trips a protective shutdown before water backs up into the unit or your ceiling cavity. This is one of the more common Adelaide callouts, particularly on systems that haven't had a filter and drain clean in a while — see our guide on how often to service your air conditioner for how to stay ahead of it.

Refrigerant and compressor faults

These are the codes that point to low refrigerant charge, a stuck valve, abnormal compressor current, or pressure outside safe operating range. This category is the most serious of the four — refrigerant is a pressurised, regulated substance, and compressor faults can escalate quickly if the unit keeps trying to run through them. This is never a DIY category, regardless of what code is showing.

When to stop troubleshooting and call a licensed technician

A few things are genuinely safe for an Adelaide homeowner to try before calling anyone: turning the unit off at the wall isolator (or the indoor unit's own power point) for a few minutes and back on to see if the code clears, and checking whether the indoor filter is visibly clogged — restricted airflow is behind a surprising number of sensor and freeze-protection codes. If either of those resolves it, you've saved yourself a callout.

Everything past that point is a job for a licensed technician, and for good reason. Opening the outdoor unit's electrical enclosure exposes mains voltage and, on some models, capacitors that hold a charge after power is switched off. Any refrigerant-related fault requires handling a sealed, pressurised system that only a licensed refrigeration technician is legally permitted to work on in Australia. Never attempt to top up refrigerant, open a sealed system, or work inside a switchboard or isolator yourself — this is exactly the kind of work our team carries out under licences BLD 274739, PGE 265145 and AU 61210, with 24/7 emergency response and same-day appointments across Adelaide as the norm, not the exception.

A code that clears with a reset but keeps coming back is also a clear signal to stop resetting and start diagnosing. Repeated resets can mask a worsening compressor or refrigerant fault until it turns what would have been a straightforward repair into a full system replacement. If you're already seeing other symptoms alongside the code — weak airflow, ice on the indoor coil, or the unit short-cycling — read our related guide on signs your air conditioner needs repair for the fuller picture.

What a Voltas technician actually does with your error code

When you call us with a code, the first thing we ask for is the brand and model number, because that determines which service documentation applies. From there, a technician uses the code as a starting point for diagnosis — checking the sensor, connection, drain path, or refrigerant circuit the code points to, confirming the fault with proper test equipment, and giving you an obligation-free quote before any repair work begins. That's true whether it's a Daikin system in Norwood or a Mitsubishi Electric ducted system out at Golden Grove — the process is the same: read the code against the real documentation, verify with testing, then quote honestly.

If the fault turns out to be more than a repair is worth — an ageing system with a failing compressor, for example — we'll tell you that plainly rather than keep patching it, and can talk you through installation options from any of the 20 brands we work with, all backed by our workmanship guarantee. For systems that are still within their useful life, our AC repair and service team and maintenance plans both reduce how often you see these codes in the first place — most drainage and sensor faults trace back to a system that's simply overdue for a clean and check.

Sources referenced for the brand examples above: Daikin U4 communication error documentation (star-supply.com), Mitsubishi Electric's official Mr Slim error code reference (mitsubishielectric.com.sg), and Panasonic fault code documentation (idealairconditioningservice.com.au). Always confirm a code against your own unit's model number before assuming it matches an example above.