The starter relay is one of the more critical parts of your ignition system, as a faulty one can mean a dead car. Nobody likes to get stuck with a car that won't start, so ensuring that your starter relay is up to snuff should be on the mind of any amateur mechanic.
It pays to first understand a bit about how the ignition system in a car works. Everyone knows that a car relies on a battery, right? However, engines are ultimately run by mechanical energy, not electrical. So how does the "juice" from the battery ultimately get the pistons firing?
The answer lies in the ignition system, and the critical starter relay. This part, also known as a solenoid, is responsible for transferring that initial electric current to the engine.
When you start your car, the ignition will send a small electric current to the solenoid. The battery will also be sending a much larger current to the starter relay. When this happens, the starter relay will close a pair of heavy contacts, which in turn passes the electric signal to the starter itself. Essentially, the "relay" term here is a good metaphor. Think of the solenoid as the middle man in a relay race, who will pass the baton (electric current) onto the next runner (the starter). The starter then physically begins turning to start the engine.
Now that you know what the solenoid does, you can see why it's important. If the starter relay is not functioning, the electric current doesn't get to the starter and the engine can't get fired up. So how do you diagnose and replace a starter relay?
First, determine the problem. Sometimes the relay has completely failed - you won't hear anything when you turn on the engine. Other times, you might hear a clicking noise. This means the contacts have failed, but you'll still need to replace the starter relay. If you look at the part, you might notice some corrosion. In some cases, this can be grinded off with sandpaper, but as a last resort you'll have to replace the relay.
Fortunately, doing so is quite easy. Follow the red, or "positive" wire from the battery to the solenoid. Make sure your hands are completely dry and the car is turned off, then disconnect the battery wires, ensuring that you do not touch the solenoid until all wires are removed.
Insert your new starter relay in the same position, then reconnect the wires in reverse order that you removed them. Voila - your car should start right up again.