iMac power supply (PSU) – won't turn on, random shutdowns
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Quick answer
Intel iMacs (21.5" and 27") have an internal power supply that typically wears out after 5-7 years — replacing it requires fully disassembling the iMac. The iMac 24" (M1/M3/M4), on the other hand, uses an external magnetic power adapter — if it fails, you just need a new adapter. We diagnose both. See per-model pricing at macmo.dk/reparation.
Two very different iMac architectures
iMac power supplies look very different depending on which model you have:
- Intel iMacs (21.5” and 27”, 2012–2020): The power supply sits inside the iMac, behind the screen. The screen is bonded in place, so replacement requires unbonding with specialist tools.
- iMac 24” (M1, M3, M4 — 2021+): The power supply is an external magnetic adapter that sits between the wall socket and the iMac. The iMac itself has no built-in PSU — just a DC input and a power IC on the logic board.
That makes repair fundamentally different per generation.
Why do Intel iMac PSUs fail?
Power supplies have one classic failure mechanism: the electrolytic capacitors dry out over time. It’s the same reason old TVs, monitors and PC power supplies fail after 5-10 years. Apple doesn’t use premium capacitors in their PSUs, so wear typically starts showing up around the 5-7 year mark.
Symptoms in order from early wear to total failure:
- The iMac feels unstable — random shutdowns once in a while
- The iMac goes to sleep and won’t wake until a hard restart
- The iMac starts but shuts down after a few minutes (the PSU can’t hold load)
- The iMac no longer starts (the PSU is fully dead)
iMac 24”: always start with the adapter
Because the iMac 24” has an external power supply, troubleshooting is very simple:
- Borrow another iMac 24” adapter from a friend or bring the iMac to us for testing. If it starts with another adapter — just a new adapter (DKK 600-800).
- Check the power cable between wall socket and adapter — it’s a standard IEC cable.
- Check the magnetic connector for dust, bent pins or mechanical damage.
- If the adapter and cable are OK but the iMac still won’t turn on, the fault is on the logic board (DC-in circuit or power IC). That requires component-level repair with us — typically DKK 1,500-3,000.
What does it cost?
Between DKK 600 and DKK 3,000 depending on model and fault:
| iMac model | What’s faulty | Price with labour |
|---|---|---|
| iMac 21.5” Intel (2012-2017) | Internal PSU + screen unbonding | DKK 1,500-2,200 |
| iMac 27” Intel (2012-2020) | Internal PSU + screen unbonding | DKK 1,800-2,500 |
| iMac 24” M1/M3/M4 — adapter faulty | New external power adapter | DKK 600-800 |
| iMac 24” M1/M3/M4 — DC-in/power IC faulty | Component-level logic board repair | DKK 1,500-3,000 |
See the current price for your specific model at macmo.dk/reparation. With us you get a free diagnosis and a fixed quote before the repair starts.
Prevention
PSU wear is unavoidable over time, but you can extend its lifespan:
- Use a good surge protector. Even a single small lightning strike can kill the PSU (internal or external adapter) instantly.
- Keep the iMac cool. Make sure there’s free airflow behind it — avoid pushing it right up against a wall or into a warm cabinet. This applies especially to Intel iMacs where the PSU sits inside the chassis.
- Clean the cooling system every 3-4 years on Intel iMacs. Blocked ventilation = higher PSU temperature = faster wear.
- Treat the iMac 24” adapter gently. Don’t tug the cable, avoid sharp bends at the connectors.
- Power the iMac off when you’re away for several days. Even though sleep is efficient, the PSU still wears slightly.
Where can I get it repaired?
We cover the full range:
- Intel iMacs: We have original or quality-approved internal PSUs for every model from 2012 onward. Disassembly requires specialist equipment (bonded screens) — it isn’t a home project.
- iMac 24” (M1/M3/M4): We test the adapter first (often a new adapter solves it). If the fault is on the logic board, we do component-level repair of the DC-in and power IC.
Diagnosis is free and no-obligation, we always agree the terms before starting on complex jobs, and you get 2 years’ warranty on repairs we carry out.
How to test whether the iMac's PSU is the problem
⏱ PT5M
- Test the wall socket. Plug another device (a lamp, phone charger) into the same socket to confirm it's working.
- Test the power cable. Borrow another IEC power cable (the same type used for PCs). If the iMac starts with the other cable, your cable is the faulty one — a much cheaper fix.
- Hold the power button for 10 seconds. Hold the power button for 10 seconds. Release. Press normally. This is a hard reset that fixes some SMC-related faults.
- Listen for a hum at startup. If you can hear a faint hum from the back but no chime, the PSU is likely failing. Bring the iMac in for diagnosis.
- Book a diagnosis with us. If nothing works, it's hardware. Diagnosis with us is free if you proceed with a repair.