iMac 24" M1 2021: Power Supply Failure

January 2025 iMac 24-inch M1 2021 Power / Hardware Repair

Overview

A small business owner from Norwest brought in their 3-year-old iMac 24-inch M1 that was experiencing random shutdowns and sometimes wouldn't power on at all. The symptoms suggested logic board failure, which would be expensive on this all-in-one design. Careful diagnosis identified the external power adapter as the culprit—a much simpler fix.

Initial Symptoms

Random shutdowns during normal use
Sometimes wouldn't power on—required multiple attempts
No warning before shutdowns (not thermal)
Power LED would flicker occasionally
Problem worsened over 2 weeks from occasional to frequent
All data intact when it did run

Diagnostic Process

iMac 24" M1 Power Architecture

The 2021 iMac 24-inch has a unique power design:

External power adapter: 143W brick with magnetic connector
Ethernet option: Built into power adapter on some models
Internal power board: Minimal—mostly distribution from adapter
M1 chip: Very efficient, low power requirements

Testing Approach

We methodically tested each component in the power chain:

Wall outlet: Stable power, tested with meter
Power adapter output: Intermittent voltage drops detected
Adapter cable: Showed resistance changes when flexed
iMac internal: Worked perfectly with known-good adapter

Root Cause

The power adapter had an internal fault causing intermittent output. When voltage dropped below threshold, the iMac would shut down instantly. The fault was in the adapter's internal cable connection—likely from thermal cycling over 3 years.

Repair Process

Solution

Replacement of the power adapter resolved all issues:

Confirmed diagnosis with test adapter—ran 48 hours without issue
Sourced replacement 143W adapter (same Ethernet configuration)
Installed and tested under various loads
Verified stable power delivery over extended period

Testing

Stress tested for 24 hours—no shutdowns
Multiple cold starts—100% success rate
Verified Ethernet through adapter working
Power delivery stable under CPU/GPU load

Outcome

Repair Status: Complete Success

Solution: Power adapter replacement only

Cost: Fraction of logic board repair cost

Turnaround: 2 business days

The business owner's iMac was returned fully functional. The simple adapter replacement resolved what initially appeared to be a serious hardware failure. The machine has run without issues since.

Key Takeaways

Power supplies fail: Even relatively new external adapters can develop faults.
Symptoms mimic worse problems: Random shutdowns often suggest logic board issues but can have simpler causes.
Systematic diagnosis saves money: Testing each component identifies the actual fault.
iMac 24" M1 power is external: Unlike older iMacs, the power supply is replaceable separately.

iMac Shutting Down Randomly?

Random shutdowns can have various causes. Proper diagnosis identifies the actual problem.

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