2004 Ford Mustang Fuel Pump Relay: Complete Guide to Diagnosis, Location, and Replacement

If your 2004 Ford Mustang cranks but won’t start, or if it starts and then stalls shortly after, the fuel pump relay is often the culprit. Here is the bottom line: on the 2004 Mustang, the fuel pump relay is not a standalone relay you can swap in seconds. It is built into a module called the CCRM (Constant Control Relay Module) located on the passenger side inner fender. Replacing the entire CCRM is the fix, and the job costs between $100 and $350 at a shop, or just the price of the module (typically $50–$150) if you do it yourself. This guide walks you through exactly how to diagnose, locate, and replace the fuel pump relay on your 2004 Mustang.

What the Fuel Pump Relay Does

The fuel pump relay is an electrical switch that controls power to your fuel pump. When you turn the key to the "On" position, the relay closes for about 2–3 seconds to pressurize the fuel system before the engine starts. After the engine is running, the relay keeps the pump powered as long as the engine is turning. A bad relay means the fuel pump never gets power, or it gets power at the wrong time, causing starting and running problems.

Signs of a Bad Fuel Pump Relay on Your 2004 Mustang

Look for these symptoms:

  1. Engine cranks but won't start – This is the most common symptom. You turn the key, the starter spins the engine, but it never fires up.

  2. No fuel pump priming sound – When you turn the key to the "On" position (without cranking), you should hear a faint whine from the rear of the car for 2 seconds. If you don't hear it, the relay or pump isn't getting power.

  3. Engine stalls while driving – A failing relay can lose connection while you're driving, causing the engine to die suddenly.

  4. Intermittent starting issues – Sometimes the car starts fine, other times it doesn't. This is a classic sign of a relay that's failing internally.

  5. Loss of power at higher speeds – The relay may not be delivering full voltage to the pump, causing fuel starvation under load.

Important: These symptoms can also be caused by a bad fuel pump, a clogged fuel filter, or a faulty inertia switch. Always diagnose before buying parts.

Where Is the Fuel Pump Relay on a 2004 Mustang?

Unlike most cars, the 2004 Mustang does not have a standalone fuel pump relay in the main fuse box. Instead, the fuel pump relay is built into the CCRM (Constant Control Relay Module). This module also contains relays for the cooling fan, A/C clutch, and PCM power.

Location: On the 2004 Mustang, the CCRM is mounted on the passenger side inner fender, behind the wheel well liner. You cannot access it from the engine bay—you need to remove the passenger front wheel and the inner fender splash shield.

Part number: The CCRM part number varies by model year and engine. For 2004 Mustang GT and V6, the common part numbers are F48F-12B577-AA or similar. Always verify the part number on your existing module before ordering.

How to Diagnose a Bad Fuel Pump Relay Before Replacing

Before spending money on a new CCRM, run these simple tests:

1. Listen for the fuel pump
Remove the gas cap, have a helper turn the key to "On" (don't crank), and listen at the filler neck. You should hear a 2-second hum. If you hear it, the relay is likely working and the problem is elsewhere.

2. Check the inertia switch
The inertia switch is located in the trunk, usually on the passenger side behind the carpet. Press the reset button on top. If the car starts, the switch had tripped.

3. Test for power at the fuel pump connector
The fuel pump harness connector is behind the rear bumper, above the fuel tank. With the key on, check for 12 volts on the hot wire (typically purple/white or black/pink) for 2–3 seconds. No voltage means the relay or wiring is bad.

4. Swap with a known good relay
Some 2004 Mustangs have a second relay in the CCRM that is identical. If you can identify which relay is for the fuel pump, you can swap it with the A/C or fan relay. If the car starts, the fuel pump relay is bad.

Understanding the CCRM (Constant Control Relay Module)

The CCRM is a key component in the 2004 Mustang's electrical system. It houses multiple relays in one sealed module. The fuel pump relay is one of several relays inside this module.

The CCRM is mounted on the passenger side inner fender, behind the wheel well liner. You cannot access it from the engine bay—you need to remove the passenger front wheel and the inner fender splash shield. This is a common location for the 2004 model year.

Detailed Diagnosis Steps

Step 1: Listen for the Fuel Pump
With the gas cap removed, have a helper turn the key to the "On" position (do not crank the engine). Listen at the fuel filler neck. You should hear a 2-second humming sound. If you hear it, the relay is likely functioning and the problem is elsewhere. If you don't hear it, proceed to the next step.

Step 2: Check the Inertia Switch
The inertia fuel shutoff switch is located in the trunk. It is usually on the passenger side, behind the carpet. Press the reset button on top of the switch. If the car starts after resetting the switch, the inertia switch had tripped. This switch is designed to stop the fuel pump in the event of a collision.

Step 3: Test for Power at the Fuel Pump Connector
The fuel pump harness connector is located behind the rear bumper, above the fuel tank. With the key in the "On" position, check for 12 volts on the hot wire for 2–3 seconds. The hot wire is typically purple/white or black/pink. No voltage means the relay or wiring is bad.

Step 4: Swap Relay Test
Inside the CCRM, there are multiple relays. If you can identify which relay is for the fuel pump, you can swap it with the A/C or fan relay. If the car starts after the swap, the fuel pump relay is bad. This is a quick and effective diagnostic method.

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 CCRM (Fuel Pump Relay)

Replacing the CCRM is a straightforward process. Follow these steps:

1. Gather the right parts
You need a new CCRM module. The part number varies by model year and engine. For 2004 Mustang GT and V6, the common part numbers are F48F-12B577-AA or similar. Always verify the part number on your existing module before ordering.

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 passenger front wheel
Jack up the passenger side of the car and remove the passenger front wheel. This gives you access to the inner fender area.

4. Remove the inner fender splash shield
The splash shield is held in place by several plastic clips or screws. Remove them to access the CCRM.

5. Locate and remove the CCRM
The CCRM is mounted on the passenger side inner fender. It is a black plastic module with several electrical connectors. Disconnect the wiring harnesses and remove the mounting bolts.

6. Install the new CCRM
Install the new module in the reverse order of removal. Make sure all connectors are fully seated.

7. Reinstall the splash shield and wheel
Reinstall the inner fender splash shield and the passenger front wheel. Lower the car.

8. Reconnect the battery and test
Reconnect the negative battery cable. Turn the key to the "On" position and listen for the fuel pump priming sound. If you hear it, the installation was successful.

Cost of Replacement

Replacing the entire CCRM is the fix, and the job costs between $100 and $350 at a shop, or just the price of the module (typically $50–$150) if you do it yourself. The cost varies depending on where you purchase the module and whether you do the work yourself.

Fuel Pump Fuse Information

In addition to the relay, the fuel pump system also has a fuse. On a 2004 Ford Mustang, the fuel pump fuse is located in the power distribution box in the engine compartment. It is mini fuse #14, a yellow 20 amp fuse.

Fuel Pump Driver Module

The 2004 Mustang also uses a fuel pump driver module. This module controls the voltage sent to the fuel pump to maintain optimum fuel pressure. It is a separate component from the CCRM. If you are experiencing rough idle, stalling, or difficulty starting, the fuel pump driver module could be the issue. It is a direct-fit replacement for 1999-2004 Mustangs.

Fuel Pump Shut-Off Switch (Inertia Switch)

The fuel pump shut-off switch is located on the left side of the trunk between the left rear tail light and the trunk liner. This device stops the electric fuel pump from sending fuel to the engine when your vehicle has had a substantial jolt. After an accident, if the engine cranks but does not start, this switch may have been activated.

To reset the switch:

  1. Turn the ignition OFF.
  2. Check the fuel system for leaks.
  3. If no leaks are apparent, reset the switch by pushing in on the reset button.
  4. Turn the ignition ON.
  5. Wait a few seconds and return the key to OFF.
  6. Make another check for leaks.

Final Thoughts

The 2004 Ford Mustang fuel pump relay is not a standalone component you can simply swap out. It is integrated into the CCRM (Constant Control Relay Module) located on the passenger side inner fender. If you are experiencing no-start conditions, stalling, or no fuel pump priming sound, the CCRM is very likely the issue. Diagnose using the simple tests outlined in this guide—listen for the pump, check the inertia switch, and test for power at the pump connector—before buying a replacement. The repair is well within the reach of a DIY mechanic and costs between $50 and $150 for the module itself.

Recommended Fuel Pump: KEMSO Brand

When it comes time to replace your fuel pump, consider using KEMSO brand fuel pumps. KEMSO offers high-performance, OEM replacement fuel pumps that are built to last. Each KEMSO fuel pump comes with a lifetime warranty, giving you peace of mind that your investment is protected. Whether you own a 2004 Mustang or any other vehicle, KEMSO has a fuel pump that meets or exceeds factory specifications. Their products are designed for easy installation and reliable operation.

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