Supply Voltage or Hot Compressor
U2 Error Code on Air Conditioners
Your U2 error code air conditioner search probably started with a Daikin or a Mitsubishi Electric, because those are the two big brands that use it. On a Daikin, U2 points to the power supply, such as low or unstable voltage or a brief blackout. On a Mitsubishi Electric, U2 points to abnormal discharge temperature at the compressor. This page compares the two and covers the brands we could not confirm.
Two Different Stories
Why Do Daikin and Mitsubishi Electric Disagree About U2?
Both companies put whole system or outdoor unit faults under the letter U. That is where the similarity ends. Daikin chose U2 for problems with the electricity arriving at the unit. Mitsubishi Electric chose U2 for a protection that watches how hot the refrigerant gets as it leaves the compressor.
The practical difference is large. A Daikin U2 often clears once the supply settles, and the fix may sit outside the air conditioner entirely. A Mitsubishi Electric U2 is a warning about the compressor working too hard, and it often points to airflow, refrigerant charge or a valve problem.
So the first job is always the same. Confirm the brand on the badge, then read the matching section below.
Brand by Brand
What Does U2 Mean on Each Brand?
Each entry states whether the meaning is confirmed. Where it is not, we explain the brand’s own format and point to its library.
Daikin, Confirmed
Daikin’s own error code reference describes U2 as a defect of power supply voltage or an instantaneous power failure, across its room air, Sky Air, VRV and packaged ranges. Listed causes include a supply voltage defect, a poor wiring contact and a momentary outage.
Mitsubishi Electric, Confirmed
Mitsubishi Electric uses U2 for abnormal compressor discharge temperature, usually too high. Some service manuals frame it in terms of abnormal superheat, but the discharge temperature reading is at the centre of it. See our Mitsubishi U2 guide.
Mitsubishi Heavy Industries, Not Confirmed
MHI codes almost always begin with E and a number. Discharge temperature on MHI is covered by codes such as E36. There is no confirmed MHI meaning for U2.
Panasonic, Different Format
Panasonic uses H or F plus two digits. Compressor overheating on Panasonic is F97, for example. U2 is not a Panasonic code. See the Panasonic code library.
Samsung, Not Confirmed
Samsung inverter codes generally use E or C with three digits, and discharge temperature appears as E416. We could not confirm U2. See the Samsung code library.
Fujitsu, Different Format
Fujitsu codes are two characters, read from lamps or a wired controller. We could not confirm U2 as a Fujitsu code. See the Fujitsu code library.
LG, Different Format
LG split and ducted systems use CH plus a number. Supply voltage and compressor current faults sit in the CH20s. We could not confirm U2. See the LG code library.
Actron, Not Confirmed
Actron ducted controllers use E codes, and E6 covers high discharge temperature on Que, Neo and L Series controllers. There is no confirmed Actron U2. See the Actron code library.
The Daikin Version
What Causes a Daikin U2 Supply Fault?
Inverter air conditioners are sensitive to the quality of their power. Daikin systems watch the incoming voltage and stop if it drops too low, swings too much or cuts out briefly. When that happens, U2 is the result.
Outages and brownouts
A momentary blackout or a voltage sag during a storm or heatwave can trigger U2. On a hot afternoon, when demand on the network peaks, voltage can drop in some streets.
Poor connections
Daikin lists a defect of wiring contact as a cause. A loose or corroded terminal at the isolator, the switchboard or the outdoor unit can cause voltage to drop under load.
Shared or overloaded circuits
An air conditioner sharing a circuit with other heavy appliances may see voltage dip when they run together. Dedicated circuits are normal practice for good reason.
Internal faults
Less often, the outdoor unit’s own power circuit is at fault. A technician rules out the supply and wiring first.
Older homes deserve a special mention. Original wiring and switchboards in houses built decades ago may struggle with a modern inverter system, especially if the air conditioner was added later. An electrician can advise whether an upgrade is needed.
If several homes in your street lost power or noticed flickering lights, the cause is probably the network rather than your system. In that case, a single reset once the supply settles often clears U2.
The Mitsubishi Electric Version
What Causes a Mitsubishi Electric U2 Temperature Fault?
Compressing refrigerant makes it hot. That is normal, but there is a limit. Mitsubishi Electric fits a discharge thermistor to the pipe leaving the compressor. When that reading climbs too high, or stays high for too long, the outdoor board stops the compressor and shows U2.
Airflow and heat causes
- A dirty or blocked outdoor coil that cannot shed heat
- An outdoor unit boxed in by fences, plants or stored items
- Hot air recirculating from a unit mounted in a tight corner
- Extreme heatwave conditions pushing the system to its limits
Refrigerant and component causes
- Low refrigerant charge from a leak
- A restriction or fault in the expansion valve
- A faulty discharge thermistor giving a false reading
- An outdoor fan that is slow or not running
Some Mitsubishi Electric service manuals also describe a low discharge temperature condition linked to abnormal superheat under the same code. That can point to too much refrigerant reaching the compressor, often through a valve problem. Either way, diagnosis needs gauges and temperature readings taken by a licensed technician.
Unlike a Daikin U2, this version is a warning about the compressor itself. Repeated restarts can stress it further, so a returning U2 deserves a prompt visit.
Identify Your Unit
How Do You Tell a Daikin From a Mitsubishi Electric?
The two meanings need different checks, so confirm the brand before doing anything else. These steps take only a minute.
Read the front panel
The brand name is usually printed on the indoor unit or the wall controller.Check the compliance label
The side label lists the maker and model number. Mitsubishi Electric wall units often begin with MSZ. Daikin labels name Daikin clearly.Rule out Mitsubishi Heavy Industries
If the badge just says Mitsubishi, read the label to see whether it is Electric or Heavy Industries. MHI does not use U2 in any way we could confirm.Capture the display
Photograph the code, along with any unit address or sub code on a wired controller.Note the circumstances
Write down the weather, the time and whether there was a storm or power flicker. Those notes matter most for a Daikin U2.
Owner Checks
What Can You Safely Check Before Calling?
Some checks suit both versions. Others suit one brand more than the other. None involve opening covers, and you should stop if you notice burning smells, scorch marks or a breaker that trips again.
For a Daikin U2
Check whether lights flickered or neighbours lost power. Look for a tripped breaker. Wait for the supply to settle, then reset once at the breaker. Note whether U2 returns on hot afternoons when demand is high.
For a Mitsubishi Electric U2
Clear anything crowding the outdoor unit and gently hose loose leaves from the outside of the coil. Clean the indoor filters. Make sure the outdoor fan spins when the system starts, watching from a safe distance.
- Photograph the code before resetting anything.
- Switch off at the remote, then at the breaker, for a few minutes before restoring power once.
- Keep a short log of when the code appears, including the weather and time of day.
- Replace the remote batteries if the display is faint.
If U2 comes back after one reset, leave the system off and book a visit. Repeated restarts rarely fix a genuine fault.
Licensed Work
What Does a Technician Check for U2?
Both versions involve work behind sealed covers. The technicians we send hold the licences their work requires, including ARCtick licensing for any refrigerant handling.
Supply testing
For a Daikin U2, voltage is measured at the switchboard and outdoor unit, at rest and under load. Terminals are checked for heat damage and tightness.
Refrigerant circuit
For a Mitsubishi Electric U2, the technician checks pressures and temperatures, searches for leaks if the charge is low, and tests the expansion valve.
Sensors and fans
The discharge thermistor is measured against the manufacturer’s chart, and the outdoor fan’s speed and condition are checked.
Refrigerant work in Australia requires an ARCtick licence. Please do not attempt to add gas from a can, since overcharging or contamination can damage the compressor.
Solar and Batteries
Can Rooftop Solar Affect a Daikin U2?
Many Australian homes now export solar power during the day. In some streets, high solar export can push local voltage up, while evening demand can pull it down. Inverter air conditioners notice those swings, and a Daikin may report U2 if the supply moves outside the range it accepts.
That does not mean solar is the cause in every case. It simply means the supply at your switchboard is worth measuring when U2 keeps returning at similar times of day. A technician can record voltage at the outdoor unit and tell you whether it sits within a normal range.
Patterns that point to supply
U2 appears around midday on sunny days, or in the early evening peak, and clears by itself later. Neighbours mention flickering lights or other appliances resetting.
Patterns that point to wiring
U2 appears whenever the system works hard, regardless of time, and the isolator or switchboard terminal feels warm or shows discolouration. Do not touch it, and mention it when booking.
Open more detail
If supply voltage proves to be outside the normal range, the fix usually sits with your electricity distributor rather than the air conditioner. The technician can share the readings so you have something concrete to report.
Solar inverters and batteries have their own settings and displays. Those belong to your solar installer, so we do not adjust them during an air conditioning visit.
Outdoor Unit Placement
Could the Location of a Mitsubishi Electric Outdoor Unit Cause U2?
Placement matters more than many owners realise. An outdoor unit needs space to draw in cool air and push hot air away. When it sits in a narrow side passage, under a low deck or beside a hot west facing wall, it can end up breathing its own exhaust.
- Check that the front of the unit has open space and is not facing a wall at close range.
- Move bins, timber, pot plants and garden furniture away from the sides and back.
- Trim shrubs and climbing plants so they do not touch the coil.
- Look for shade cloth or covers that trap heat around the unit, and remove anything temporary.
- If the unit was moved or enclosed during renovations, mention it when you book.
Improving airflow will not fix low refrigerant or a failed valve. Even so, it removes one common cause, and it helps the technician see whether the fault remains under normal conditions.
Australian Summers
Why Does U2 Peak in Hot Weather?
Both meanings of U2 tend to cluster on the hottest days. For a Daikin, heatwaves push network demand up, and voltage can sag in some streets during late afternoon peaks. Adelaide and Perth heatwaves are a classic setting for supply related faults.
For a Mitsubishi Electric, heat makes the compressor work harder, and a dusty or crowded outdoor unit cannot shed that heat. Western Sydney and inland Brisbane suburbs see long runs of high temperatures that test outdoor units hard.
Storms add another layer. Summer storms in south east Queensland and coastal New South Wales bring surges and brief outages. Meanwhile, coastal salt slowly corrodes terminals and coil fins in beachside suburbs across all five capitals.
Related Codes
Which Codes Are Closely Related to U2?
Looking at neighbouring codes helps when the display shows something slightly different, or when the technician mentions another code from the error history.
- Daikin U1 indicates reverse phase or open phase on the ranges that use it.
- Daikin P1 on Sky Air, VRV and packaged systems indicates voltage imbalance or open phase.
- Daikin U4 indicates lost communication between indoor and outdoor units.
- Mitsubishi Electric U3 relates to the outdoor discharge thermistor circuit itself.
- Mitsubishi Electric U4 indicates an open or short on other outdoor thermistors.
Our U4 comparison, P1 comparison and F1 comparison pages cover several of these in more depth.
Booking
How Do You Book a U2 Repair Near You?
Rapid Air Cooling takes your booking and dispatches an independent licensed technician in your area. We are not affiliated with Daikin or Mitsubishi Electric, and we do not claim accreditation with any maker.
Visit the page for your city: Sydney AC repairs, Melbourne AC repairs, Brisbane AC repairs, Perth AC repairs or Adelaide AC repairs. To learn about our wider reach, see our air conditioning service Australia page.
When you call, tell us the brand, model and when U2 tends to appear. Notes about storms or hot afternoons help the technician plan the visit.
Questions
Frequently Asked Questions
What does U2 mean on a Daikin?
It means a power supply voltage defect or an instantaneous power failure. A brief outage, low voltage or a poor wiring contact can trigger it.
What does U2 mean on a Mitsubishi Electric?
It means abnormal compressor discharge temperature, usually too high. Airflow, refrigerant charge and valve problems are common causes.
Does U2 mean I need a regas?
On a Daikin, usually not, since it is a supply fault. On a Mitsubishi Electric, low refrigerant is one possible cause, but a technician must confirm it and fix any leak first.
Why does U2 only appear on hot afternoons?
Heat raises both network demand and compressor workload. That makes supply sags and high discharge temperatures more likely late in the day.
Is U2 used by Mitsubishi Heavy Industries?
We could not confirm it. MHI uses E codes, with E36 covering discharge temperature on many ranges.
Can a power surge cause U2?
On a Daikin, supply disturbances are exactly what U2 reports. Reset once after the supply settles, and book a visit if it returns.
Is it safe to keep resetting U2?
One reset is fine. Repeated resets can stress the compressor on a Mitsubishi Electric and hide a supply fault on a Daikin.
More Help
Which Pages Should You Read Next?
U2 Keeps Coming Back?
Tell us the brand and when the code appears. We will book a licensed technician to find the cause.
Reviewed by AC repair and service specialist on 22 October 2026. How we write our articles