Mitsubishi Winter Help
Mitsubishi Air Conditioner Not Heating
A Mitsubishi air conditioner not heating usually announces itself on the coldest night of the week. Before assuming the worst, it helps to know how these reverse cycle systems warm a room, pause for defrost and protect themselves. This guide focuses on Mitsubishi Electric units, notes where Mitsubishi Heavy Industries models behave differently, and shows you what to check safely before calling a licensed technician.
Check the Badge
Is Your Unit Mitsubishi Electric or Mitsubishi Heavy Industries?
Two separate manufacturers sell air conditioners under the Mitsubishi name in Australia. They share a history, yet they build their own systems, remotes and fault lists. Knowing which one you have avoids a lot of confusion later.
Mitsubishi Electric wall splits usually carry model numbers starting with MSZ, such as the popular MSZ-AP range and the slimline MSZ-EF Designer series. Mitsubishi Heavy Industries, often shortened to MHI, makes the SRK wall splits along with its own ducted ranges. Look at the front badge, then confirm with the compliance label on the side of the indoor unit.
Mitsubishi Electric
MSZ wall splits, PEA and similar ducted units, cassettes and floor consoles. Check codes are short letter and number pairs, such as P8 or U2.
Mitsubishi Heavy Industries
SRK wall splits plus ducted and packaged systems. Fault codes on wired controllers start with E and a number, such as E36 or E57.
Most heating advice on this page applies to both. Where the brands differ, we say so clearly.
Symptoms
How Does a Mitsubishi Heating Fault Usually Show Up?
Owners describe heating trouble in a handful of ways. Matching your experience to one of them is the quickest route to a likely cause.
The fan will not start after switching to heat
Mitsubishi units hold the indoor fan until the coil warms up. This prevents a cold draught. On a frosty morning the delay can stretch out, which many people mistake for a fault.
Air flows but feels lukewarm
Settings, airflow and refrigerant charge sit at the top of the list here. A dirty filter alone can make warm air feel tepid.
Warmth stops and starts
Regular pauses in cold weather are usually defrost cycles. However, pauses that come with a blinking lamp or a code point to a protection trip.
Cooling works, heating does not
This pattern often involves the four way reversing valve, a sensor or the control board, rather than the compressor itself.
Defrost Explained
Why Does My Mitsubishi Stop Blowing Warm Air in Winter?
To heat your home, the outdoor unit absorbs warmth from outside air, even when that air is cold. As a result, its coil can drop below freezing and collect frost. Frost acts like a blanket, so the system must clear it every so often.
During defrost, the indoor fan stops and the outdoor fan goes quiet. The system briefly reverses, sending hot refrigerant to the outdoor coil to melt the ice. You may notice a hiss or a soft thump as valves change over, followed by water and even steam leaving the outdoor unit.
Afterwards, the system flips back to heating and the indoor fan restarts once the coil is warm again. In mild weather this might happen rarely. On damp, near freezing nights, though, it can happen frequently, and the room may feel slow to warm as a result.
Both Mitsubishi Electric and MHI units defrost in this general way. The exact timing and the indicator behaviour differ between models, so check your user manual if you want to see what your unit displays during defrost.
- Normal: short pauses, steam from the outdoor unit, heating returning on its own.
- Not normal: thick ice that never clears, constant defrosting with little warmth, or a code appearing.
Likely Causes
What Stops a Mitsubishi Heating Properly?
Work down this list from the top. The left column covers things an owner can sort out. The right column lists faults that need tools, training and in some cases a refrigerant licence.
Owner level causes
- Mode left on Cool, Dry or Fan from the summer
- Set temperature lower than the room, so the unit has nothing to do
- Auto mode switching between heat and cool unexpectedly
- Flat remote batteries sending incomplete signals
- Filters packed with dust, lint or pet hair
- Weekly timer or app schedule turning the unit down
- Curtains or furniture blocking the return air
- Outdoor unit hemmed in by plants, bins or a cover
Technician level causes
- Refrigerant loss from a leaking joint or coil
- Reversing valve stuck or its coil failed
- Faulty room, pipe or outdoor thermistor
- Outdoor fan motor weak or stopped
- Linear expansion valve not regulating flow
- Indoor or outdoor control board damaged by a surge
- Compressor overheating and tripping protection
- Ducted zone motor stuck shut
On MHI systems, a closed or partly closed service valve after an installation or relocation can also starve the system. That is a technician job too.
Features That Matter
Can a Mitsubishi Setting Make It Seem Like It Is Not Heating?
Several clever features can make a working Mitsubishi seem underpowered. Checking them costs nothing.
Room sensor positioning
Many Mitsubishi Electric models use an i-see sensor to read the temperature of surfaces and people in the room. Depending on settings, the unit can direct warmth away from where you sit, or ease off when it thinks the room is occupied comfortably.
Economy and quiet modes
Low power and quiet settings reduce fan speed and output. They are ideal on mild days but can leave a room cold during a winter chill.
Wifi control and schedules
Wifi enabled units can be controlled from a phone. A family member may have set a schedule or changed the mode without anyone noticing.
Vane direction also matters. Warm air rises, so heating works best with the vanes angled downward. If the vanes point at the ceiling, warmth can pool up high while your feet stay cold.
Safe Checks
What Should You Try Before Booking a Mitsubishi Repair?
Each of these checks is safe for a homeowner. You will not need to remove any screws, and you will stay well clear of refrigerant lines and live terminals.
Set the remote to Heat, not Auto
Choose Heat directly, then set the temperature a few degrees above the room. Auto can switch modes on its own, which confuses the diagnosis.Swap the remote batteries
New batteries rule out partial signals. If the display still looks faint or the unit ignores commands, read our article on an air conditioner remote not working.Clean the filters gently
Open the front panel and slide out the filters. On models with a secondary filter, follow the manual, because some electrostatic panels should be dry cleaned only.Open the vanes downward
Point the airflow toward the floor so warm air can rise through the room.Free up the outdoor unit
Clear away leaves and anything stored nearby. Leave any ice alone and let defrost deal with it.Restart at the breaker
Turn the system off at the switchboard for several minutes, then restore power. Our guide to resetting common air conditioning faults explains why this helps.Allow time to warm up
Give the unit at least ten to fifteen minutes before judging it, especially on frosty mornings.
A breaker that trips again immediately should stay off. That points to an electrical fault, and repeated resets can cause damage.
Fault Codes
Which Mitsubishi Codes Can Appear When Heating Fails?
Mitsubishi Electric and MHI use different code systems, so always match the code to your brand. Our Mitsubishi error codes hub covers both lists and how to retrieve them.
P6, freeze or overheat protection (Mitsubishi Electric)
In heating, the indoor coil can run too hot when airflow is restricted. A dirty filter is a frequent trigger. See Mitsubishi P6.
P8, pipe temperature (Mitsubishi Electric)
The indoor board expects the coil pipe temperature to change in a certain way after start up. If it does not, P8 can appear. Read Mitsubishi P8.
P1 and P2, indoor sensors (Mitsubishi Electric)
A faulty intake or pipe sensor confuses the board about how warm things really are. Learn more on Mitsubishi P1 and Mitsubishi P2.
U2 and U4, outdoor protection (Mitsubishi Electric)
High discharge temperature or an outdoor thermistor fault can halt the compressor. See Mitsubishi U2 and Mitsubishi U4.
E6 and E9, communication or other faults
On Mitsubishi Electric these relate to indoor and outdoor communication. On MHI, E6 is an indoor coil sensor fault and E9 is drain trouble. Read Mitsubishi E6 and Mitsubishi E9.
E36 and E57 (Mitsubishi Heavy Industries)
E36 relates to compressor overheat or high discharge temperature, and E57 to refrigerant shortage or a closed service valve. See Mitsubishi E36 and Mitsubishi E57.
Local Climate
How Do Australian Winters Affect Mitsubishi Heating?
Heating loads vary widely by city, and so do the faults that surface. Long, cold evenings in Melbourne keep reverse cycle units running for hours. Frost in outer areas such as the Dandenongs foothills or Melton forces frequent defrosts, which exposes low refrigerant and tired sensors.
In Adelaide, clear winter nights and frosty Hills suburbs push units hard after months of summer cooling. Summer dust left in filters and coils then reduces heating output. Meanwhile Perth has damp winter fronts and coastal salt air, which slowly corrodes outdoor fins and lowers efficiency.
Sydney is milder near the coast, yet western suburbs like Penrith get genuinely cold nights. In Brisbane, heating is used only briefly, so a fault may hide for most of the year and appear during a cold snap.
Summer storms also leave a mark. A surge that weakens a control board may not show up until winter, when the unit is first asked to switch into heating.
Technician Visit
What Will a Technician Test on a Mitsubishi That Is Not Heating?
A technician will confirm the fault in Heat mode first. Some problems never show during a cooling test, so testing both modes matters. Then the checks follow the heat from the outdoor coil to your room.
- Retrieve stored check codes from the controller or by the self check procedure.
- Compare outlet air temperature with the room temperature.
- Check the outdoor coil for ice, dirt and corrosion.
- Test the outdoor fan motor and confirm it runs at the right speed.
- Confirm the reversing valve changes position when commanded.
- Measure refrigerant pressures and pipe temperatures.
- Leak test flare connections and coils if the charge looks low.
- Measure thermistor resistance against the manufacturer’s tables.
- Inspect wiring, terminals and the link between indoor and outdoor units.
- On MHI systems, confirm both service valves are fully open.
Refrigerant work in Australia must be carried out by an ARCtick licensed technician. We only dispatch technicians who hold the licences needed for the job in front of them.
Keep or Stop
Can You Safely Keep Running a Mitsubishi With No Heat?
If the unit runs quietly, the breaker holds and no code appears, using it in Cool or Fan mode is generally harmless. You just will not get heating until the cause is fixed.
Stop using it if you notice a burning smell, the breaker trips, the outdoor unit knocks or grinds, or a protection code keeps returning. Thick ice that stays on the outdoor coil for hours is another reason to switch off, because the compressor may be struggling.
Fine to leave on standby
No warmth but no other symptoms. Use a safe portable heater in the meantime and book a visit.
Turn off at the breaker
Repeating codes, burning smell, tripping breaker, harsh mechanical noises or ice that will not clear.
Ducted and Multi Head
Why Do Mitsubishi Ducted and Multi Head Systems Struggle to Heat?
Larger systems add a few extra ways for heating to go wrong. On a Mitsubishi ducted system, warm air must travel through the roof before reaching each room. A split or disconnected duct can dump that warmth into the ceiling space, leaving one room cold while others feel fine.
Zone controls also matter. If most zones are closed, the system may run at reduced capacity or cycle more often. Similarly, a blocked return air grille starves the indoor unit of air, so the coil overheats and output drops.
Multi head systems have their own quirk. One outdoor unit serves several indoor heads, and they must all agree on the operating direction. If one head is set to Cool while another asks for Heat, the outdoor unit cannot satisfy both. Setting every head to Heat usually resolves the conflict.
Cold Weather Limits
Does Very Cold Weather Reduce Mitsubishi Heating Output?
All reverse cycle systems lose some heating capacity as outdoor temperatures fall. There is simply less heat in very cold air to collect. On top of that, defrost cycles become more frequent, and each one briefly pauses warm airflow indoors.
In practice, this means a unit that comfortably heats a room on a mild evening may struggle on a frosty dawn. That is especially true if it was sized tightly for the space, or if the room has large windows and little insulation.
Signs it is a capacity limit
Heating works well on milder days, the unit runs constantly in cold snaps, no codes appear and the outdoor unit clears its frost normally.
Signs it is a fault
Heating is weak even on mild days, the outdoor coil stays iced, a code appears or performance has dropped compared with previous winters.
If heating has been weaker than in past winters, treat it as a possible fault rather than a capacity limit. A technician can measure output and compare it with what the system should deliver.
Good to Know
How Can You Avoid a Cold Surprise Next Winter?
A few small habits keep Mitsubishi systems heating reliably.
Open more detail
Run a short heating test in autumn. Finding a problem before the first frost means a calmer repair.
Clean filters regularly once heating season starts. Closed windows mean more dust stays indoors.
Keep a clear zone around the outdoor unit, and make sure defrost water can drain away rather than pooling and refreezing.
Mitsubishi Electric sets servicing expectations in its warranty terms, so check your warranty booklet and keep records of professional services.
For broader heating advice across all brands, read our guide on a reverse cycle air conditioner not heating. If your Mitsubishi will not start at all, see Mitsubishi AC not turning on, and for summer issues read Mitsubishi air conditioner not cooling properly.
Book a Visit
How Do You Arrange a Mitsubishi Heating Repair?
We are a booking and dispatch service, not a Mitsubishi dealer or manufacturer. We line up licensed technicians who work on Mitsubishi Electric and MHI systems in major cities.
When you call, have the model number, any code and the results of your checks ready. Browse the Mitsubishi air conditioning page for the range we book work on, the AC troubleshooting library for other symptoms, or our locations page.
Learn more about our air conditioning service Australia network, or send a written enquiry through the contact page.
Questions
Frequently Asked Questions
Why does my Mitsubishi wait before blowing air in heat mode?
It is warming the indoor coil first. This avoids blowing cold air at you, and the delay is longer on very cold mornings.
Why does steam come from the outdoor unit?
That is frost melting during a defrost cycle. Steam is harmless and the unit returns to heating shortly afterwards.
Do Mitsubishi Electric and MHI codes mean the same thing?
No. They are different manufacturers with separate code lists. E6, for example, means a communication fault on one and a sensor fault on the other.
Can the i-see sensor make my room feel colder?
Depending on settings, it can aim airflow or adjust output based on where people are. Check the sensor and airflow settings in your manual.
My Mitsubishi cools but will not heat. Is it the compressor?
Usually not. If cooling works, the compressor runs. The reversing valve, sensors or board are more common culprits for heating only faults.
How long should a defrost cycle take?
Generally only a few minutes. If defrost seems constant or the outdoor unit stays iced, book a check.
Can I add refrigerant myself?
No. Handling refrigerant requires an ARCtick licence, and a leak must be found and fixed before any regas.
Mitsubishi Still Not Warming Up?
If your checks have not brought the heat back, give us a call. We will book a licensed technician to test your Mitsubishi in Heat mode and find the fault.
Reviewed by AC repair and service specialist on 22 October 2026. How we write our articles