Fuse doesn't sit in fuse box tightly

First you have to determine if the fuse is loose in the terminal or the terminal is loose in the fuse box. It is common for the terminals to be loose in the fuse box. What is important is it grips the fuse terminal tightly. If that's where it is loose you can usually go alongside it with a thin metal pick to push the side together. The less-desirable way to make an emergency repair is to twist the terminal on the fuse a little. That's less desirable because if the fuse blows and has to be replaced in the future, you may not know or remember that was done so you'll have that loose connection problem again. Doing that will also spread the fuse box terminal some more.

The only terminals I have ever had to completely replace were those where my students were doing learning exercises on the same donated vehicles year after year. Sometimes they poked in them too hard to take voltage readings and the meter probe spread them until they made poor contact with the fuse.

In the worst case you can take the fuse box apart and and remove one terminal or a group that are riveted together, then replace the bad one or squeeze it tighter.

Disconnecting the battery will insure you don't damage anything, but I hesitate to recommend that today. There are too many manufacturers building tricks into their cars to force you to go back to the dealer to have multiple computers unlocked. Simply replacing a bad battery can lead to all kinds of expensive repairs.

To avoid the chance of causing that heartache, just watch that any metal tool you use only touches the fuse box terminal, and no other metal part of the car at the same time. Actually, you have to really mean it to cause a problem, but it's worth mentioning. I had the most problems with stretched fuse box terminals on GM products, so I got to be pretty familiar with fixing them. If your car uses the same type, you'll see each one is a thin strip of metal about 1/8" wide, and folded over on top. That fold becomes squeezed too tightly, then the gap where the fuse terminal slides in becomes too wide. A small pick with a right-angle hook on the end works best. Poke it alongside, then under that fold, then try to pull it up gently. One end of that metal strip is attached solidly in the box. By pulling up, the pick will try to bend the fold, and that will stretch the unanchored end and close the gap.

If that doesn't work, use a straight pick and stick it in behind the terminal. That will bend the entire terminal closer to its mate and close the gap.

If the terminal is beyond hope, consider sliding another piece of thin metal into it, then slide the fuse in. The extra strip will take up the gap and force the terminal to make better contact with the fuse terminal. That extra strip has to have a wider area or a little part of it bent over on top to keep it from falling through into the fuse box. By this time, if you had to resort to this, the next repair is to replace the terminal. Most fuse boxes are made up of two or more parts of the housing that are snapped together. Once opened up, you'll see either a single wire going to the bad terminal, or it will be one of many on a strip of metal connecting them together. If it's on a strip, all of them must be removed together. For a single wire, just remove that one. The people at all GM dealers' parts departments have large kits of terminals for all their models. They can find what you need by the car's model, year, and application, or you can take the old terminal and they can match it up.

If you're more industrious, or as cheap as I am, you can visit a pick-your-own-parts salvage yard, and harvest a handful of terminals. This will give you the opportunity to experiment and figure out how to release the terminals. There's usually a hard-to-see plastic finger, or hook, molded into the hole, that must be pulled back, then the terminal can be pulled out by the wire.