Nothing is broken. The system is doing exactly what it is designed to do, and the symptom points nowhere near the cause: the room that stops working is often not the room whose thermostat caused it.
What your brand calls it
Seven of the brands we catalogue publish a code or status for this, under seven different names:
| Brand | What you'll see |
|---|---|
| GREE | E7 |
| Samsung | C142 (mixed operation) |
| LG | CH 07 |
| Mitsubishi Electric | MXZ mode conflict |
| Daikin | mode conflict - the non-priority head enters standby |
| Midea | mode conflict, often shown as -- or ---- |
| ACiQ | mode conflict, often shown as -- or ---- |
Note the Midea and ACiQ behaviour especially: a head that simply displays dashes and does nothing looks far more like a failure than an error code does.
Safe checks: match the modes
- Check every indoor head on that outdoor unit - including ones in rooms nobody is using. A single head left on Cool in a spare bedroom is enough to stop the rest of the house heating.
- Set them all to the same mode - all Heat, all Cool, or all Off. Turning unused heads off entirely is usually simplest.
- Wait several minutes. These systems do not re-coordinate instantly.
- The stopped head should resume on its own once the conflict clears.
If you have a head you rarely use, leaving it off rather than on AUTO avoids this permanently. AUTO is the usual culprit, because a head in AUTO can decide it wants cooling on a mild afternoon while the rest of the house is still heating.
When to stop and call a technician
If every head is set to the same mode and one still will not run, you are past this. Look up the code your unit is actually showing in the error-code library - the same display can mean something entirely different on a different brand - and work from there.
Call a technician if the affected head shows a numbered fault code after the modes are matched, if it never runs regardless of mode, or if it runs but blows ambient air continuously.
The design detail worth knowing before you buy
Single-mode operation is a property of most residential multi-zone mini splits, not a defect in yours. If simultaneous heating and cooling in different rooms genuinely matters - a south-facing home office that needs cooling while bedrooms need heat - that is a heat recovery system, which is different equipment at a different price, and it is worth raising with a contractor before installation rather than discovering it afterwards.
Comparing multi-zone options? The product finder shows zone counts and capacities across the systems we have verified data for.