iMac Not Turning On? Power Supply Issues Explained

Symptoms of a failing iMac power supply

• Press the power button - no fan spin, no startup chime, no display
• No diagnostic LED visible through the back lower-right vent slot
• Faint clicking or buzzing sound from the back of the iMac
• Burning electronic smell (act fast - capacitor failure)
• Intermittent: works for 5 minutes then dies, then works again after cooling
• Reboots randomly under load (the dying PSU can't deliver current)

Which iMacs have this issue

By far the most affected: iMac 27″ Retina 5K (2017, 2019, 2020) with the Delta-made internal PSU. The 2017 model is now 8-9 years old and hitting end-of-life on bulk capacitors at scale. We see 4-6 of these per week in our Dubai workshop.

Also affected, less commonly:

• iMac 27″ Late 2015 - slightly different PSU but same failure mode
• iMac 21.5″ 4K (2017, 2019) - smaller PSU, similar issues
• iMac Pro (2017) - separate enterprise-grade PSU, fails differently

Not affected: iMac M1 (2021), M3 (2023), M4 (2024) - Apple Silicon iMacs use a much smaller external 143W power brick. PSU failures are rare.

How to diagnose - the LED count trick

Apple built four internal diagnostic LEDs onto every Intel iMac 27″ logic board. They are visible through the bottom right of the back vent slot if you peer in with a flashlight at the right angle. Each LED indicates a power milestone:

• LED 1 only: PSU is delivering 12V trickle power (PSU is alive). If this LED is OFF, the PSU is dead - that's your fault.
• LED 1 + 2: SMC has woken and confirmed 12V main rail. If LED 2 fails to light, SMC or main board fault.
• LED 1 + 2 + 3: GPU is alive and communicating. Failure here = GPU or video cable issue.
• All 4 LEDs: Display received signal. iMac should now show the boot sequence.

For the "iMac won't turn on at all" scenario, we're checking LED 1. If it doesn't light with the power cable plugged in (you don't even need to press the power button to see LED 1 - it's always lit on a healthy iMac), the PSU is the fault.

Why this happens - component aging

Inside the iMac PSU are bulk electrolytic capacitors rated for roughly 5,000-8,000 hours at 105°C. UAE summer indoor temperatures push the iMac's internal PSU temperature consistently above 70°C - accelerating capacitor degradation.

By year 5-7 the electrolyte inside these capacitors has dried out enough that they can no longer hold their rated voltage. The PSU's primary side (the part connected to the 240V mains) typically fails first because it sees the highest stress. Result: the iMac stops powering on.

We see a clear seasonal pattern - peak failure rate in August and September after summer heat soak. Customers who keep their iMac in air-conditioned rooms year-round generally see PSU life of 8-10 years; customers whose iMac sits in unconditioned office storerooms see 5-6 years.

Repair cost - AED 400 labour + PSU part

• Labour: AED 400 (full disassembly: glass, LCD, logic board removal, PSU swap, reassembly with new adhesive seal)
• PSU part - iMac 27″ 2017/2019: AED 250-350 (refurbished tested) or AED 450 (new old stock)
• PSU part - iMac 27″ 2020: AED 350-450 (less common, costs more)
• PSU part - iMac 21.5″ 4K: AED 200-300
• Adhesive re-seal kit: AED 50 (included)
• Total typical: AED 700-850 same-day

Compare to Apple Store: out-of-warranty iMac 27″ "Service" cost is AED 2,899 (whole logic board + PSU swap whether needed or not). Our component-level approach saves AED 2,000+ for the same outcome.

How long the repair takes

Same-day if you drop off before 11am and we have the PSU in stock. Bench time is 120-150 minutes (the iMac display is bonded to the chassis with strong adhesive that takes time to release without cracking). We re-seal with new adhesive and do a 4-hour stress test before handing back. Returned with a 90-day warranty on the PSU and the workmanship.

Can you DIY this? (No - high voltage, sealed unit)

Strongly no, for two reasons:

1. Lethal voltage stored after unplugging. The bulk capacitors inside the PSU hold 400V DC and can stay dangerously charged for hours after you unplug the iMac. Touching the wrong terminal during disassembly can cause cardiac arrest. We discharge through a bleeder resistor before any work.
2. Display adhesive is unforgiving. The iMac display is glued to the aluminum chassis. DIY removal without a heated separation table almost always cracks the LCD or chips the front glass. A AED 250 PSU repair becomes a AED 1,200 display + PSU repair.

iFixit sells a heated table and discharge tools for AED 1,500. If you're an experienced electronics technician with a fume hood, sure. For everyone else, the AED 700 we charge is genuinely the right answer.

Apple's stance - won't repair out-of-warranty

Apple Stores and AASPs in UAE will quote the "iMac whole logic board + PSU + display" module replacement at AED 2,899-3,599 because that's the official Apple service tier. They won't isolate the PSU. If your iMac is older than 6 years, Apple may decline service entirely under "vintage and obsolete products" policy.

Our shop is one of about 4 in Dubai that does iMac PSU swap as a standalone service. See our iMac repair page for all iMac models we service.