An S21, S21+ or S21 Ultra that runs warm is normally doing something Samsung lists as ordinary: charging, restoring data, installing updates, long games or navigation, or sitting in the sun. The phone defends itself by dimming, slowing, pausing charging and closing apps. Heat that comes with fast drain on a five-year-old phone points to a worn battery, and heat that stays after everything is closed and unplugged is a reason to contact Samsung. The chip depends on region: Snapdragon 888 in the US and Canada, Exynos 2100 elsewhere.
Differential
Likely causes of “Samsung Galaxy S21 family gets hot or shows a temperature warning”, and how to tell them apart
- 1
Demanding apps
What separates it Warm and dimming during games, maps or long video
How to confirm Samsung lists gaming and GPS use; stop and see if it cools
- 2
Direct sun or a warm car
What separates it It heats fast outdoors because of direct sun or a warm car
How to confirm Samsung's range is 32 to 95 degrees Fahrenheit ambient; shade it
- 3
A faulty or unsuitable charger
What separates it The phone is hottest while it sits on the charger and cools down once unplugged
How to confirm Try an approved charger on a hard flat surface
- 4
A restore or update running
What separates it The phone stays warm for days after a data transfer, restore or large update
How to confirm Samsung says restores raise temperature; leave it on Wi-Fi to finish
- 5
A worn cell
What separates it The phone is warm in light use and the battery also drains faster than it used to
How to confirm Run the battery test; module repair includes a cell
Before you touch a setting
Record this first — changing settings destroys the evidence
- Which S21 model and its SM- number
- What the phone was doing and the message it showed
- Whether another charger and Wi-Fi change it
- The One UI version, the battery result and the room temperature
Try in this order
0 of 7 completedTake the load off
ReversibleTake the load off, by unplugging, closing apps, removing the case and letting the phone cool fully.
Read the message
ReversibleCooling down, Power off device, Device powered off automatically and Device overheating all mean the phone is protecting itself, and normal use returns after cooling.
Change charger and surface
ReversibleAn approved charger and cable, a hard flat surface, no metal between phone and pad.
Switch off idle features
ReversibleSwitch off idle features, by turning Wi-Fi, GPS and Bluetooth off when unused, and lowering the brightness.
Let a restore finish
ReversibleKeep it on Wi-Fi and out of a case until the transfer ends, because indexing and restoring can keep it warm for a while.
Run the battery test
ReversibleRun the battery test, in Samsung Members, Support, and a poor result on a phone of this age points to a new cell.
Contact Samsung if it lingers
ReversibleContact Samsung if it lingers, and if it stays hot with nothing running, ask for a service check.
Model-specific
What is true of this model and not of its siblings
- Chips by region
- Snapdragon 888 (US, Canada, China, Japan) or Exynos 2100 (international), with 8 GB RAM on the S21 and S21+ and 12 GB on the Ultra.
- Displays
- 6.2 inch (S21), 6.7 inch (S21+) and 6.8 inch (Ultra) Dynamic AMOLED 2X panels; the Ultra is 3200 by 1440.
- Part numbers
- Samsung uses the SM- number rather than a part number.
- Tools and supplies
- Shade, a hard flat surface, a second charger and a soft cloth.
- Repair cost
- If a worn cell is the cause, module repair with battery is $199, $229 or $269; screen-only is $174, $199 or $229.
- Other models this page answers
- These steps also fit the Galaxy S21+, Galaxy S21.
Stop boundary
When this stops being a self-service repair
- !If it stays too hot to hold after unplugging and closing apps, contact Samsung
- !If it shuts down to cool while idle, run diagnostics and book a check
- !If the back is swollen or smells scorched, stop and go to a Samsung center that day
Evidence ledger
Sources behind this answer
Exact questions
Common questions
Is warm normal on an S21?+
Yes in the cases Samsung lists: charging, transfers, updates, long games or navigation, and hot surroundings.
What will it display?+
Cooling down, Power off device, Device powered off automatically or Device overheating.
Does the plastic back matter?+
When is it a battery fault?+
When it is warm doing little and also drains fast; run the battery test.
Why does the phone get hot while charging and using it?+
Charging generates heat on its own, and heavy use adds more. Samsung lists both as normal heat sources. Avoid gaming or video calls while charging, take the case off and charge on a hard flat surface so the heat can leave, and use an approved charger.
Does a screen protector or case affect heat?+
A thick or insulating case holds heat, which matters during charging and games. Samsung's guidance to remove heat sources includes the case. If the phone cools faster without it, use a thinner case or leave it off while charging.
When should I worry about heat?+
Worry when the phone is too hot to hold with nothing running, when it shuts itself down repeatedly, or when the back bows or smells scorched. Those are cases for a Samsung service center rather than more troubleshooting at home.
Can an old battery cause overheating on its own?+
It can add to it. A worn cell has to work harder to supply the same power, and heat and fast drain together are the classic pair. If the phone runs warm doing very little and empties quickly, run the Samsung Members battery test before changing anything else, since a new cell is the fix.