How do I work out what is wrong with a device?

A general method that applies to any device: isolate, simplify, change one thing at a time.

Difficulty: beginner
Time: 30 min
Read: 2 min

Short answer

Change one thing at a time and test after each change. The universal method is to narrow down whether the problem is the device, the connection, the software or the input — and most faults are found within four or five tests.

Most technical troubleshooting is the same procedure regardless of the device. Isolating variables systematically finds a fault far faster than trying likely fixes at random, and it tells you what the problem was.

What you'll need

Patience (optional)
Something to test with (optional)

Step by step

Describe the problem precisely.What happens, when it started, what changed just before. 'It stopped working' is not diagnosable; 'no sound since the update, only in one app' nearly is.
Check whether it affects everything or one thing.One app or all apps, one device or all devices, one website or all websites. This single question halves the problem immediately.
Restart it properly.A genuine restart, not sleep. It resolves a surprisingly large share of faults by clearing stuck processes and states, and it costs a minute.
Change one thing at a time.Test after each change. Changing three things and finding it works tells you nothing about what was wrong or what to do next time.
Substitute components to isolate.A different cable, a different socket, a different device, a different account. Swapping one element at a time identifies which one is faulty.
Simplify the setup.Remove hubs, docks, extensions and accessories. Get it to the simplest possible configuration that should work, then add things back.
Look at what changed just before it broke.An update, a new device, a house move, a power cut. Correlation in time is the most useful diagnostic clue there is.
Search the exact error message.In quotes, with the device model. Almost every fault has been encountered before, and the exact wording finds it far better than a description.

Tips

Write down what you have tried. Half an hour in, it is easy to lose track and repeat things, particularly when frustrated.
If you have made several changes and it is worse, undo them all and start again methodically. Compounding changes makes faults harder to find.
Know when to stop. Two or three hours on a device problem is usually the point to seek help or accept a replacement.

Common mistakes

Changing several things at once — If it works, you do not know why, and if it does not, you have added variables. One change, then test.
Not checking whether it is device-wide or app-specific — It is the single most informative question and it takes ten seconds. It determines everything you do next.
Skipping the restart because it seems too simple — It resolves a large share of faults by clearing stuck states. It is the first step for good reason.

Questions people ask

What is the first thing to try when technology stops working?

Restart it properly, then establish whether the problem affects one app or everything, and one device or all of them. Those two steps resolve or halve most problems.

How do I find out what is causing a problem?

Change one thing at a time and test after each change, substituting cables, devices and accounts to isolate which element is at fault. Also look at what changed just before it broke.