How to run a diagnostic test on iMac?

Shut the iMac down, disconnect external devices except the keyboard, mouse, display, and power, then turn it on and immediately hold the D key. iMacs introduced in June 2013 or later launch Apple Diagnostics; older models launch Apple Hardware Test. Hold Option + D instead to run the internet-based version.

How do I diagnose what's wrong with my Mac?

Start with Apple Diagnostics or Apple Hardware Test by holding D at startup; note any reference codes it reports for logic board, RAM, or storage faults. For disk issues, run First Aid in Disk Utility. For software problems, boot into Safe Mode by holding Shift and check whether the issue persists.

How to update El Capitan on Mac?

Apple no longer publishes updates for El Capitan itself. If your Mac supports a newer release, download a later macOS installer such as High Sierra from Apple's support site and run the upgrade. Back up with Time Machine first, since a failed upgrade on decade-old hardware can be hard to recover.

Test on macOS El Capitan: Hardware Diagnostics & Browser Testing Online

Q: Is macOS El Capitan still usable?

El Capitan (OS X 10.11) still boots and runs, but Apple stopped shipping security updates for it years ago, Safari is capped at version 11, and most modern apps and browsers no longer support it. It is fine for checking how legacy Safari renders a site, but it is not safe as a daily work machine. For web testing, a cloud El Capitan machine avoids keeping vulnerable hardware online.

Run Apple Hardware Test on an El Capitan Mac, or skip the old hardware and test your website on real El Capitan browsers online, including Safari 11, for free.

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Hardware Diagnostics

How to Run Apple Hardware Test (AHT) on macOS El Capitan

If your El Capitan Mac is misbehaving, first rule out a hardware fault with Apple's built-in diagnostics. Macs introduced in June 2013 or later run Apple Diagnostics; older Intel Macs run Apple Hardware Test. The startup steps are the same:

Disconnect all external devices except the keyboard, mouse, display, Ethernet cable, and power adapter.
Shut the Mac down completely, then press the power button.
Immediately press and hold the D key until the diagnostics screen or a language selection appears.
To run the internet-based version instead, hold Option + D. The Mac loads diagnostics from Apple's servers over Wi-Fi or Ethernet.
Run the test and note any reference codes it reports. Codes point to specific components such as the logic board, RAM, or storage.

Troubleshooting

Apple Diagnostics on Older Intel Macs

On the 2009-2012 Intel Macs that commonly run El Capitan, the D key shortcut sometimes does nothing. Work through these fixes:

Use the original install media. Macs from around 2010 or earlier shipped with Apple Hardware Test on the gray Applications Install Disc 2. Insert that disc, restart, and hold D to launch AHT from the disc.
Check the network for Option + D. The internet-based version needs a working Wi-Fi or Ethernet connection at boot. Prefer wired Ethernet on older Macs, since some Wi-Fi security modes are not supported before macOS loads.
Use a wired keyboard. Third-party wireless keyboards may not register startup key combinations. A wired Apple keyboard is the most reliable way to send the D key at boot.
Remove a firmware password. Startup key combinations, including diagnostics, are blocked while a firmware password is enabled. Disable it in Recovery first.

Legacy Coverage

Is macOS El Capitan Still Usable for Web Testing?

Not as a daily machine. OS X 10.11 stopped receiving Apple security updates years ago, Safari is frozen at version 11, and current releases of Chrome and Firefox no longer support it. Modern JavaScript and CSS features frequently break on it, which is exactly why teams still need to check their sites against it.

You do not need to keep a decade-old Mac alive for that. TestMu AI gives you on-demand cloud machines running El Capitan-era Safari alongside 10,000+ current browser, OS, and real device combinations, so one platform covers both your legacy checks and your modern coverage.

Test on El Capitan Browsers Online

Live sessions on real El Capitan machines running Safari 11, Chrome, Firefox, and Opera, plus tunnel access for local builds.

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LIVE TESTING

Live Testing in the Cloud

Spin up a real macOS El Capitan machine in your browser and interact with your site the way users did on legacy Chrome, Firefox, Opera, and Safari.

Start a Free Live Session

Real El Capitan machines, streamed to your browser
Legacy and latest browser versions side by side
Mark bugs to Jira, Asana, Trello, or Slack mid-session

SAFARI 11

Test on Safari 11

Safari 11 is the last Safari on El Capitan and the trickiest browser of its era. Verify layouts and JavaScript against it without keeping an old Mac alive.

Test on Safari 11

The exact WebKit build your legacy visitors use
Compare against modern Safari on newer macOS
Screenshots and video logs for every session

LOCAL TESTING

Tunnel to Localhost

Test locally or privately hosted builds on El Capitan cloud machines through the TestMu AI Tunnel over secure shell, before anything reaches production.

Read the Tunnel Docs

Secure shell tunnel from your machine to the cloud
Works for plain HTML, CSS, PHP, or Python files
Same tunnel powers CI runs via GitHub Actions

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