Early startup kernel panic

Since I updated my iMac to macOS Sierra 10.12.4, it can’t start up properly. Whenever it tries to start up, I hear the normal chime, then shortly afterwards there’s text on the display about a “panic” warning, and it freezes. How should I try to fix it next?

This is a “kernel panic” occurring during early startup, which is possibly the result of problems in the update. It’s a general term for any error from which the operating system can’t recover, and so it shuts down the system to prevent any further damage.

Although some of the latest MacBook Pros experienced similar issues because of a Touch Bar firmware update in 10.12.4, there shouldn’t have been anything to cause this in iMacs. This could be a coincidental memory problem: kernel panics are still most likely to be the result of hardware failure, so try running Apple Diagnostics or Hardware Test by holding on your keyboard at startup.

If your hardware passes the test, reset the SMC and NVRAM. If that doesn’t help, you may need to reinstall macOS Sierra, which is probably best done from the Recovery system, entered by holding during startup. Further details are at bit.ly/updprob.

The most common cause of kernel panics remains hardware faults, such as memory module failure. It’s always worth running hardware tests in this situation.