04 Mustang Fuel Pump Relay: The Complete Guide to Diagnosis, Location, and Replacement

The bottom line: If your 2004 Ford Mustang GT or V6 is cranking but won't start, or if it starts and then stalls, the fuel pump relay is often the culprit. On the 2004 Mustang, the fuel pump relay is not a standalone relay you can swap in seconds—it's 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:

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

Step-by-Step: How to Replace the Fuel Pump Relay (CCRM) on a 2004 Mustang

Here's the process to replace the entire CCRM:

Tools and supplies needed:

  • Replacement CCRM (correct part number for your year/model)
  • 5mm and 5.5mm sockets
  • Ratchet and extensions
  • Jack and jack stands
  • Flashlight
  • Safety glasses

Step 1: Disconnect the battery
Always disconnect the negative battery cable before working on electrical components. This prevents shorts and shocks .

Step 2: Jack up the passenger front and remove the wheel
Use a jack to lift the passenger front of the car and support it on jack stands. Remove the passenger front wheel.

Step 3: Remove the inner fender splash shield
The splash shield is held in by plastic push pins and a few 10mm bolts. Remove them to expose the inner fender area. The CCRM is mounted on the frame rail, just behind the wheel opening .

Step 4: Disconnect the wiring harness
The CCRM has a large wiring harness connector. Use a 5mm socket to loosen the bolt that secures the connector to the module. Once loose, pull the connector straight off .

Step 5: Remove the CCRM mounting bolts
The module is held by two 5.5mm bolts. Remove them and pull the CCRM off the bracket .

Step 6: Install the new CCRM
Mount the new module in the same position, tighten the bolts, reconnect the wiring harness, and tighten the connector bolt.

Step 7: Reinstall the splash shield and wheel
Put the splash shield back in place, reinstall the wheel, and lower the car.

Step 8: Reconnect the battery and test
Reconnect the negative battery cable. Turn the key to "On" and listen for the fuel pump priming sound. If you hear it, start the engine and take it for a test drive.

Time estimate: 30 minutes to 1 hour for a DIY mechanic .

Cost of Replacing the Fuel Pump Relay on a 2004 Mustang

  • DIY cost: The CCRM module costs between $50 and $150, depending on whether you buy OEM or aftermarket. You'll need no special tools beyond basic sockets .
  • Shop cost: A mechanic will charge $100 to $350 total, including parts and labor .
  • Relay-only cost: If you have a different model where the relay is separate, a replacement relay costs $12 to $80 .

Common Mistakes to Avoid

1. Replacing the fuel pump when the relay is bad
This is the most common mistake. If the relay isn't sending power, a new pump won't fix anything. Always test for power at the pump connector first .

2. Buying the wrong CCRM
V6 and V8 CCRMs are different, and 2001–2004 models differ from 1994–2000. Check the part number on your existing module before ordering .

3. Forgetting to reset the inertia switch
If your car won't start after a bump or minor accident, the inertia switch may have tripped. Always check this before buying any parts .

4. Not disconnecting the battery
Working on the CCRM with the battery connected can cause a short that damages the module or the PCM. Always disconnect the battery first .

Why You Should Consider Upgrading Your Fuel System

If you're replacing the fuel pump relay because your Mustang has fuel delivery issues, it's worth considering the overall health of your fuel system. A failing relay can strain the fuel pump over time, and a weak pump can cause similar symptoms.

Recommendation: Upgrade to a KEMSO Fuel Pump

For reliable, long-lasting performance, consider replacing your fuel pump with a KEMSO Fuel Pump. KEMSO is a trusted brand known for manufacturing high-performance, OEM replacement fuel pumps that meet or exceed factory specifications. Every KEMSO fuel pump comes with a lifetime warranty, giving you peace of mind that your fuel system is built to last. Whether you're restoring a classic Mustang or building a daily driver, KEMSO delivers the durability and reliability you need.

Visit https://www.kemsoracing.com/ to browse their full lineup of fuel pumps for Mustangs, motorcycles, ATVs, and more. With KEMSO, you're not just buying a part—you're investing in confidence for the long haul.

Final Thoughts

The 2004 Mustang fuel pump relay is part of the CCRM module, located on the passenger side inner fender. If your Mustang cranks but won't start, or stalls unpredictably, diagnosing the relay system should be your first step. Listen for the pump priming sound, check the inertia switch, and test for power at the pump connector before buying any parts. Replacing the CCRM is a straightforward DIY job that takes about an hour and costs between $50 and $150. If the fuel pump itself needs replacing, go with a KEMSO Fuel Pump for a high-performance, lifetime-warranted solution. Visit https://www.kemsoracing.com/ to find the right pump for your 2004 Mustang.


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