Fuel Pump Relay: What It Does, How It Fails, and What to Do Next

When your car won’t start but the battery’s fine, the problem might not be the fuel pump—it could be the fuel pump relay, a small electrical switch that tells the fuel pump when to turn on. Also known as a fuel pump control relay, it’s the quiet hero that powers your engine’s fuel supply. Without it, no fuel reaches the injectors, and your car just sits there, clicking but not turning over.

This relay is part of your car’s electrical system, the network of wires, fuses, and switches that control power flow to critical components. It works with the fuel pump, the device that pushes gasoline from the tank to the engine, and connects to the engine control unit, the car’s main computer that decides when to activate the fuel pump. If the relay fails, the ECU can’t signal the pump—even if the pump itself is perfectly fine. That’s why so many people replace expensive fuel pumps only to find out the relay was the real issue.

Relays fail for simple reasons: heat, age, or moisture. Under the hood, they’re exposed to engine temperatures that can warp the internal contacts. Over time, the metal inside gets worn down from constant switching. A bad relay might click once when you turn the key, then go silent. Or it might work only when it’s cool—until the engine warms up and it quits again. You can test it by swapping it with another relay in the fuse box (like the horn or AC relay) to see if the car starts. If it does, you’ve found your culprit.

Don’t assume a no-start problem is always the fuel pump. Many mechanics skip the relay because it’s cheap and easy to miss. But if you’ve checked the battery, the starter, and the fuses, the relay is the next logical step. It’s one of the few car parts you can replace in under ten minutes with just a screwdriver. And it costs less than a coffee at most garages.

When you do replace it, make sure you get the right one. Relays aren’t universal—each car model uses a specific type, often labeled with a number like 555 or 37220. Check your owner’s manual or the label on the old relay. Using the wrong one can fry your fuel system or even cause a fire.

What you’ll find below are real-world stories from drivers who thought they needed a new fuel pump—until they tested the relay. You’ll see how to diagnose the issue without tools, what symptoms to watch for, and why some cars are more prone to relay failure than others. Whether you’re a weekend mechanic or just trying to get your car to start on a cold morning, these guides cut through the guesswork and give you the facts you need to fix it right the first time.