Direct answer
For the Samsung Galaxy S21 Ultra, high workload, charging, signal search, environment and a failing battery must be separated. First, stop charging/workload, remove the case and move to room temperature.
What this covers
- Samsung Galaxy S21 Ultra (SM-G998*)
- Current Galaxy family; confirm the complete SM identifier, region and One UI version before repair or firmware instructions.
- The specific fault this record addresses: gets hot or shows a temperature warning
What it does not cover
- ×The same symptom on a different Samsung product line, which has its own record and its own cited source
- ×An on-screen error code whose meaning has not been checked against this exact model's manual
- ×Live electrical, gas, sealed-system, internal battery-cell or structural repair — a technician boundary, not a self-service step
Differential
What actually causes this, and how to tell them apart
These are ordered by how often each one is the answer, not by how easy it is to try. The point of the middle column is that two causes producing the same headline symptom rarely produce the same detail — that detail is what tells you which one you have before you change anything.
- 1
Direct sun or an enclosed hot space
What separates it It heats without being used — on a dashboard, a windowsill, or in a pocket against the body in summer. A phone in direct sun absorbs far more than it generates.
How to confirm Move it into shade and see whether it recovers within minutes. Environmental heat is the most common cause and the one people least suspect.
- 2
Sustained heavy work rather than a fault
What separates it It heats during gaming, video recording, navigation with the screen on, or a long video call — and cools within minutes of stopping. This is the processor doing what it was asked.
How to confirm Note what was running. Warmth during heavy use that resolves on stopping is normal; heat at idle is not, and the two need different investigations.
- 3
Fast charging, particularly with a case on
What separates it It heats on the charger and is normal otherwise. Fast charging generates real heat, and a thick case traps it against the back.
How to confirm Charge without the case and on a hard surface rather than a bed or sofa. A large difference confirms it — and a phone charging under a pillow is doing permanent capacity damage.
- 4
An app misbehaving in the background
What separates it It is warm at idle with the screen off, and the battery breakdown shows one app well above the rest. A stuck process holds the processor awake continuously.
How to confirm Check the battery usage breakdown. Idle heat with one app dominating is the finding, and no thermal advice substitutes for removing the cause.
- 5
Poor signal driving the radio at full power
What separates it Heat tracks location — hot at a particular building, normal elsewhere — and the battery drains fast at the same time. Transmitting at maximum power generates heat.
How to confirm Compare a weak-signal location against a strong one. Heat and drain moving together with location is a radio cause.
- 6
Post-update indexing
What separates it It ran hot for a day or two after a major update and is easing. Photo and search indexing are genuinely heavy work that happens once.
How to confirm Note the update date. Give it a few charge cycles before treating it as a fault.
- 7
The battery itself is failing
What separates it Heat at idle with nothing running, no environmental cause, and possibly swelling or a lifting back or screen. This is the one entry on the list that is dangerous.
How to confirm Inspect the back and screen for lifting and the phone for any distortion. This is a stop condition, not a diagnosis to continue past.
Before you touch a setting
Record this first — changing settings destroys the evidence
Every item below stops existing the moment a reset, re-pair or settings change is made. Written down first, they are what separates the causes above; recovered afterwards, they are guesswork.
- When the heat appears: during use, while charging, in the sun, or at idle. These four are different faults and the answer costs nothing.
- Where on the body of the phone it is hot — the top back near the cameras, the middle, or across the whole back — since charging heat and processor heat sit in different places.
- What was running at the time, and what the battery usage breakdown attributes it to.
- Whether a case was fitted and what surface it was resting on while charging.
- Whether the phone recovers within minutes of stopping, or stays hot.
- Any warning message shown, and whether the phone throttled, dimmed or shut down.
- Whether the back or screen has lifted at any edge — this is checked with a fingernail along the seam, and it changes everything else.
Establish when the heat appears
ReversibleDuring heavy use, on the charger, in the sun, or at idle. Warmth during gaming that resolves in minutes is the phone working; heat at idle with nothing running is not. These four have nothing in common and the answer takes one question.
Check the back and screen for lifting
ReversibleRun a fingernail along the seam. A back or screen that has started to lift means the battery is swelling, which is a stop condition — not something to configure around. Do this before any other step, because it changes what you should do next.
Move it out of sun or an enclosed hot space
ReversibleA phone on a dashboard or a windowsill absorbs far more heat than it generates. Environmental heat is the most common cause and the least suspected, and recovery within minutes of moving into shade confirms it.
Remove the case and charge on a hard surface
ReversibleFast charging generates real heat and a thick case traps it against the back. Charging on a bed, a sofa or under a pillow is genuinely damaging — heat permanently reduces capacity, so this is not only a comfort issue.
Read the battery usage breakdown for idle heat
ReversibleHeat with the screen off and nothing running means something is running. The breakdown names it, and no amount of thermal management substitutes for removing the cause.
Compare heat by location
ReversibleA phone transmitting at maximum power because it cannot find service generates real heat, and the drain rises with it. Heat and drain moving together with location is a radio cause rather than an app one.
Allow a few cycles after a major update
ReversiblePhoto and search indexing after a major update is genuinely heavy work that happens once. Judging thermal behaviour in the first days after an update measures the indexing rather than the phone.
Let it cool fully before judging anything
ReversibleA hot phone throttles, which changes how it performs and how it charges. Any comparison made while it is still hot measures the throttling rather than the change you made.
Model-specific
What is true of this model and not of its siblings
Generic advice for the product line fails here. Each item below is a property of this specific model family that changes which of the steps above apply to you.
- Model scope
- SM-G998*. Current Galaxy family; confirm the complete SM identifier, region and One UI version before repair or firmware instructions.
- Tier — flagship Ultra (S Pen support, NO slot)
- The S21 Ultra supports an S Pen but has NO built-in storage slot — you buy the S Pen and a case with a pen holder separately. This is different from the S22 Ultra onward, which integrate the pen. So 'where does the S Pen go' is: it does not store in the phone on this model.
- One UI / Android
- One UI over Android; nearing the end of its major-update window. DeX support.
- eSIM + physical SIM, IP68
- eSIM and physical SIM, IP68, fast + wireless charging.
- Symptom boundary
- This record covers Samsung Galaxy S21 Ultra when it gets hot or shows a temperature warning. A different symptom on the same hardware, or this symptom on a different Samsung product line, has its own record with its own causes and its own cited source.
Stop boundary
When this stops being a self-service repair
Continuing past any one of these costs more than the repair saves — in safety, in warranty, or in evidence a technician needs.
- !The back or screen has lifted, the phone is distorted, or the battery is visibly swollen. Stop using it, stop charging it, and do not compress or puncture it.
- !It is hot at idle with nothing running, no environmental cause and a normal battery breakdown.
- !It has shut down on a temperature warning more than once in ordinary use.
- !There is a smell of heat, any discolouration, or the phone has been hot enough to be uncomfortable to hold.
Evidence ledger
Sources behind this answer
Visible sources support the visible claims. Home Product Support records the publisher, the fact used and the review date; community reports can suggest an issue but do not become a published fact on their own.
Where the manufacturer's article stops. Samsung publishes this as "Cooling down popup appears on Samsung phone or tablet". Explains the "Device cooling down" temperature warning, what the phone throttles when it overheats, and how to cool it down. Probed 2026-08-21: title-verified. What that article does not carry is the model boundary — it is written for a product line, and SM-G998* is the scope applied here. Where a control label, terminal, indicator pattern or reset sequence differs on your unit, the manual for that exact model is the authority, not this page and not the article.
Exact questions
Common follow-ups
Where do I store the S Pen on my Galaxy S21 Ultra?+
You don't store it in the phone — the S21 Ultra supports an S Pen but, unlike the S22 Ultra and later, it has no built-in S Pen slot. Samsung sold a separate S Pen and a case with a pen loop for it. So there is no internal dock to charge it and the basic S Pen has no Bluetooth button features (the pricier S21 Ultra 'S Pen Pro' added those). Integrated, stored S Pens start with the S22 Ultra.
Is it normal for my phone to get warm?+
During sustained heavy work — gaming, video recording, navigation with the screen on, a long video call — yes, and it should cool within minutes of stopping. Warm while charging is also normal. Hot at idle with nothing running is not, and that distinction is the whole diagnosis.
It gets hot while charging. Should I worry?+
Fast charging generates real heat, and it matters far more where the phone is than how long it charges. A phone charging on a bed, a sofa or under a pillow, in a thick case, is doing permanent capacity damage. On a hard surface with the case off, warmth is expected.
It got hot in the car and shut down.+
That is the phone protecting itself, and it worked. A dashboard in sun reaches temperatures far beyond anything the phone generates, and it absorbs rather than produces the heat. Repeated exposure permanently reduces battery capacity even though the phone recovers each time.
It is hot but I am not using it.+
Then something is using it. Check the battery usage breakdown with the screen off — a stuck background process holds the processor awake continuously and produces exactly this. If the breakdown attributes nothing and there is no environmental cause, that is a different and more serious finding.
Does a case make it worse?+
Yes, particularly a thick or insulating one during fast charging or heavy use. The back of the phone is a heat path, and a case is deliberately in the way of it. Removing the case during charging or gaming is a real, measurable change.
Can heat permanently damage the battery?+
Yes, and this is the reason to take it seriously. Heat is the main driver of capacity loss besides charge cycles, and it is not reversible — a phone that has cooked in a car repeatedly will not return to its original runtime even once it is cool.
My phone got hot after an update.+
Photo indexing, search rebuilding and app optimisation all run once after a major update, and they are genuinely heavy. Give it a few charge cycles before treating it as a fault — you are measuring the indexing, not the phone.
Should I put it in the fridge to cool it down?+
No. Rapid cooling causes condensation inside the phone, which is a far worse problem than the heat. Move it into shade, take the case off, stop what it is doing, and let it come down on its own.
The screen is lifting at one edge.+
Stop. That is the battery swelling and pushing the display out, and it is a stop condition regardless of how well the phone otherwise works. Do not charge it unattended, do not compress it, and do not attempt to press it back.
Does this answer apply to every Samsung model?+
No. It is scoped to SM-G998*. Current Galaxy family; confirm the complete SM identifier, region and One UI version before repair or firmware instructions.
What should I record before troubleshooting overheating or hot phone?+
Record the complete model identifier, exact message or indicator, software/firmware where visible, the operating stage and what still works.
Why is a factory reset not the first step?+
A reset can erase accounts, networks, maps, schedules or preferences without distinguishing overheating, hot phone. The ordered checks preserve that evidence.
When should I stop and use qualified service?+
Power off for swelling, smoke, odor, extreme heat or repeated warnings at idle.
Answer for the exact device and software version shown above.