How to fix Minecraft "Exit Code 1" crash

The problem

How to find and fix the cause of Minecraft closing with exit code 1, usually a mod error, the wrong Java version or a mod built for another loader.

A mod threw an error while loading. This is the most common cause in modded games.
The launcher is running the game with a Java version it does not support.
A mod built for a different Minecraft version or a different loader (for example a Fabric mod in a Forge instance).
Broken or outdated graphics drivers.

Exit code 1 only says "the game stopped with an error". The real reason is in the log. Open your instance folder and read logs/latest.log, or the newest file in crash-reports.
Search the log for the first line that contains Exception, Error or Caused by. The mod name usually appears in the lines right below it.
Paste the log into the error finder to match it against known causes.

Read the crash report and note the mod or file named near the first Caused by line.
Update that mod to the newest file for your exact Minecraft version and loader, or remove it.
Check that every mod in the mods folder is for the same loader and Minecraft version as the instance.
Make sure the instance uses the right Java version. See Java versions for Minecraft.
Update your graphics drivers from the GPU maker's website.
If you cannot find the cause, move half of your mods out of the folder, launch, and repeat with the half that crashes until one mod is left.

Create a fresh instance with only the loader installed. If that fails too, the problem is Java, drivers or the launcher, not your mods.
Back up and then delete the instance's config folder; a corrupted config file can stop a mod from loading.

All Minecraft versions, Java Edition. Most reports come from modded instances.

Is exit code 1 a problem with my PC?

Rarely. In most cases it is a mod or Java problem that the log explains. Hardware is the last thing to check.

Why does the launcher not show the real error?

The launcher only sees that the game process stopped. The game writes the reason to latest.log and the crash report.