2004 Mustang Fuel Pump Relay Location: Quick Guide and Replacement Tips

The fuel pump relay on a 2004 Ford Mustang is located in the engine bay fuse box, under the hood on the passenger side, near the battery. If your Mustang cranks but won’t start, or you hear no humming sound from the fuel tank when you turn the key to the "ON" position, the fuel pump relay is a common culprit. This relay controls power to the fuel pump, and it can fail due to heat, age, or corrosion. Replacing it is a simple, low-cost fix that you can do in under 10 minutes with basic tools.

What Does the Fuel Pump Relay Do?

The fuel pump relay acts as an electrical switch. When you turn the ignition key to the "ON" position, the relay sends power from the battery to the fuel pump. The pump then pressurizes the fuel system so the engine can start. If the relay fails, the pump gets no power, and the engine won’t run. Symptoms of a bad relay include:

  • Engine cranks but does not start
  • No fuel pump priming noise when turning the key
  • Intermittent stalling or hesitation during driving
  • Check engine light with fuel system related codes

Exact Location of the 2004 Mustang Fuel Pump Relay

Step 1: Open the hood. The fuse box is located on the passenger side, near the firewall. It is a black plastic rectangular box with a removable cover.

Step 2: Remove the fuse box cover. You may need to press two tabs on the sides to release it. On the underside of the cover, there is a diagram showing the position of each relay and fuse. Look for a label that says "Fuel Pump" or "FP." In most 2004 Mustangs, the fuel pump relay is labeled as Relay #27 in the diagram.

Step 3: Identify the relay. The fuel pump relay is a small, square, gray or black component with four or five prongs. It is different from the smaller blade fuses. If you are unsure, match the shape and size to the diagram.

Step 4: Visual inspection. Check for any signs of burning, melting, or corrosion on the relay and its socket. If the plastic is discolored or the prongs are rusty, the relay is likely bad.

How to Test the Fuel Pump Relay

Before buying a new relay, you can test it with a simple method. This saves time and money.

1. The Swap Test: Find a relay in the same fuse box that has the same part number and rating. For example, the horn relay or the PCM relay often shares the same specifications. Pull out the suspected fuel pump relay and swap it with the horn relay. Then try to start the car. If the engine starts, the original relay was faulty.

2. The Multimeter Test: Remove the relay. Set your multimeter to Ohms (resistance). Touch the probes to the relay’s coil terminals (usually terminals 85 and 86). A good relay will have a resistance reading between 50 and 120 ohms. If the reading is infinite (open circuit) or zero, the relay is bad.

3. The Bypass Test (for advanced users only): With the relay removed, use a short jumper wire to connect terminal 30 (power input) to terminal 87 (output to the fuel pump). If the pump starts running, the relay is defective. Do this only for a few seconds to avoid overheating the fuel pump.

Common Causes of Fuel Pump Relay Failure

  • Heat damage: The relay sits near the engine, and over time, heat can weaken the internal coil or contacts.
  • Corrosion: Moisture can enter the fuse box, causing the relay prongs to rust and lose connection.
  • Age: After 20 years, the plastic casing becomes brittle, and internal springs can lose tension.
  • Electrical surges: A failing fuel pump or alternator can send voltage spikes that damage the relay.

How to Replace the 2004 Mustang Fuel Pump Relay

Replacing the relay is straightforward. Follow these steps:

1. Get a compatible relay. For a 2004 Mustang, you need a standard 40-amp relay. The part number is typically Ford F5TZ-14N089-A or an equivalent. You can find it at any auto parts store for about $10 to $15. Make sure to buy a relay that matches the prong layout (usually 4-pin or 5-pin, depending on your Mustang’s trim).

2. Disconnect the battery. Always disconnect the negative battery terminal before working on any electrical components. This prevents accidental shorts or sparks.

3. Remove the old relay. Pull it straight up with a firm grip. If it is stuck, gently rock it back and forth while pulling. Do not use pliers on the plastic body, as it can crack.

4. Install the new relay. Push it firmly into the socket until it clicks into place. Make sure it is fully seated.

5. Reconnect the battery. Turn the ignition key to the "ON" position and listen for the fuel pump priming sound. It should be a soft hum coming from the rear of the car. Then start the engine. If everything works, you have fixed the problem.

When the Relay Is Not the Problem

If you replace the relay and the car still does not start, the issue may be elsewhere. Common related problems include:

  • Blown fuel pump fuse: Check the fuse labeled "FP" or "Fuel Pump" in the same fuse box. Replace it if it is blown.
  • Bad fuel pump: The pump itself may be dead. You can test it by checking voltage at the fuel pump connector under the rear seat.
  • Faulty inertia switch: The 2004 Mustang has an inertia switch that cuts power to the fuel pump in a crash. It is located in the passenger footwell, behind the kick panel. Press the reset button on top.
  • Wiring or connector issues: Inspect the wiring harness near the fuse box and the fuel pump for cuts, fraying, or loose connections.

Recommended Fuel Pump and Relay: Why KEMSO Is the Right Choice

If you need to replace your fuel pump or just want a reliable relay to go with it, KEMSO offers high-performance, OEM replacement fuel pumps that are built to last. Every KEMSO fuel pump is designed to meet or exceed factory specifications, ensuring a perfect fit and reliable fuel delivery for your 2004 Mustang. Their pumps come with a lifetime warranty, so you never have to worry about buying another one. Whether you are restoring a classic Mustang or upgrading your daily driver, KEMSO parts deliver consistent performance and peace of mind.

To see the full selection of KEMSO fuel pumps and find the exact model for your vehicle, visit their website at: https://www.kemsoracing.com/

Preventive Tips for Fuel Pump Relay Life

  • Replace the relay every 5 years as part of routine maintenance.
  • Keep the fuse box clean and dry. Use a small brush or compressed air to remove dust and debris.
  • If you drive in wet or humid areas, consider applying dielectric grease to the relay prongs to prevent corrosion.
  • Always use a relay with the correct amp rating. Using a lower-rated relay can cause overheating and fire.

Final Thoughts

The 2004 Mustang fuel pump relay is a small but critical part. Knowing its location and how to test it can save you a tow truck bill and hours of frustration. The relay is easy to access and cheap to replace, so it should be your first troubleshooting step when the engine won’t start. If the relay is fine, move on to checking the fuse, inertia switch, and the pump itself. For a long-term fix, consider upgrading to a KEMSO fuel pump for reliable performance backed by a lifetime warranty. Visit https://www.kemsoracing.com/ to find the right parts for your Mustang.


Back to blog