My brake lights stopped working and my horn wasn't working either.

 

According to the test the mechanic did the fuses were fine. He replaced them anyway and the brake lights started working and so did the horn. If you can't trust a fuse tester, what can you trust?

It's possible that the original fuse didn't make proper contact with the fuse holder. It's also possible that the wiring behind the fuse panel is a little flaky, and that there is a loose wire back there.

 

My vote goes to "fuse holder". These things are simply tab connectors, and the connector may be loose in the holder, allowing itself to be pushed out of the holder when you insert the fuse. That's happened to me.

Don't feel too bad. My heater stopped working a week ago. Tested all of the fuses three damn times and then did a quick look over. A about four hours into it (dash board apart, wires every where, 25F outside) I needed to get the car ready for work. Plug in the relays and comp, test the fuses one more time only to find that the ingition 15A has gone out!!!!!!!!!!!!! Heater workers.:-\

But why would the fuse tester "light-up", meaning the fuse was passing a signal, yet not until the fuse was replaced did the brake lights and horn start working?

 

A loose wire should have meant that the fuse would not have passed voltage, right?

But why would the fuse tester "light-up", meaning the fuse was passing a signal, yet not until the fuse was replaced did the brake lights and horn start working?

 

A loose wire should have meant that the fuse would not have passed voltage, right?

That depends on how the fuse was tested. Normally, you have to pull the fuse to test it.

That depends on how the fuse was tested. Normally, you have to pull the fuse to test it.

 

The mechanic had a some type of continuity tester with a small light bulb on one end that lit up when he touched it to either sides of the fuse holder.

It could depend on which side had the bad connection. If the bad connection was on the protected side, or load, then the voltage would show fine through the fuse test points but the load would be dead. Also, the tester would not be a significant load so even if the problem was on the battery side and the load not applied to the circuit it may look fine with the tester.

Fuses can sometimes get hairline cracks that can break the connection... hardly visible to the eye. Quickly looking you could easily overlook one.

Is there a "failsafe" fuse tester, or do they all have flaws?

Is there a "failsafe" fuse tester, or do they all have flaws?

Try an actual load tester. One that puts a load on the circut when it tests it. The little fuse tester I have uses a tiny watch battery... doubt it's putting much of a load on the ciurcut when it tests.

You could make your own load tester by using something like a brake light. This will cause enough current draw to check for a bad connection problem.

Go to Radio Shack and pick up a simple 8-function test meter for $15. It does AC volts in three ranges, DC volts in three ranges, current, and resistance. Use the resistance on the fuse, but take the fuse out so no juice is in it because testing resistance on a live circuit will kill the meter. If the resistance buries the needle, then the fuse is blown. If it shows something normal, then it's not the fuse.

The cheaper way is probably that of the over paid mechanic. Replace the first fuse and move that one down the line.