In this macOS Tahoe (2026) tutorial, I show you the quickest way to put your Mac into Apple Diagnostics mode so you can check for hardware issues without installing anything. The idea is simple: start clean, boot into the right startup screen, and let Apple’s built-in tool run its tests. I keep it straight to the point—because when your Mac is acting up, you don’t want a 20-minute detour. I walk you through the exact steps: shut down your Mac, disconnect external devices, then press and hold the power button (or Touch ID) until you see the startup options screen and “Options.” On Apple Silicon Macs, you then hold Command (⌘)-D to launch Apple Diagnostics. From there, follow the on-screen prompts—sometimes you’ll need to pick a language and make sure you’re on Wi‑Fi using the menu in the top-right. When the test finishes, you’ll get results with one or more reference codes, which is the key takeaway: those codes tell you what Apple Diagnostics found and what to do next.