2004 Ford Mustang Fuel Pump Relay: The Complete Guide to Diagnosis, Location, and Replacement
The bottom line: If your 2004 Ford Mustang GT or V6 cranks 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:
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 . According to the 2004 Mustang owner’s manual, 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 . To reset the switch: Turn the ignition OFF, check for fuel system leaks, reset the switch by pushing in on the reset button, turn the ignition ON, wait a few seconds, return the key to OFF, and check again for leaks .
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 Confusion: Two Different Locations Online
You may find conflicting information online about the 2004 Mustang fuel pump relay location. Some sources claim it’s in the engine bay fuse box as a standalone relay . However, this is incorrect for the 2004 model year. The 2004 Mustang specifically uses the CCRM system.
Why the confusion exists: Earlier Mustang models (1994-1998, for example) used a traditional standalone fuel pump relay in the fuse box. The 2004 model, however, integrates the relay into the CCRM module. If you read a guide that says “pull the fuel pump relay from the fuse box,” it’s likely written for a different year.
The fuel pump fuse is different: While the relay is in the CCRM, the fuel pump fuse is located in the power distribution box under the hood. On the 2004 Mustang, mini fuse #14 is a yellow 20-amp fuse for the fuel pump . Always check this fuse first before replacing the CCRM.
Step-by-Step CCRM Replacement on a 2004 Mustang
If your diagnostics point to a bad CCRM, here’s how to replace it:
Tools you’ll need:
- Jack and jack stands
- Socket set (10mm, 8mm)
- Flathead screwdriver
- Trim removal tool (optional)
Step 1: Disconnect the battery – Always disconnect the negative terminal before working on electrical components.
Step 2: Jack up the passenger side – Safely lift the front of the car and place it on jack stands. Remove the passenger front wheel.
Step 3: Remove the inner fender splash shield – You’ll find several plastic push pins and maybe a few 8mm bolts holding the inner fender liner in place. Remove them and pull the liner back.
Step 4: Locate the CCRM – It’s mounted on the inner fender wall, about halfway up, near the firewall. It’s a black plastic box with multiple wiring harnesses plugged into it.
Step 5: Disconnect the wiring harnesses – Each harness has a locking tab. Press the tab and pull the connector straight out. Label each connector with tape and a marker so you know which goes where. There should be 3-4 connectors total.
Step 6: Remove the mounting bolts – The CCRM is held in by 2-3 10mm bolts. Remove them and pull the module out.
Step 7: Install the new CCRM – Position the new module, install the bolts, and reconnect the wiring harnesses. Make sure each connector clicks into place.
Step 8: Reinstall the splash shield and wheel – Put the inner fender liner back, install the push pins, and put the wheel back on.
Step 9: Reconnect the battery and test – Turn the key to "On" and listen for the 2-second fuel pump prime. Then start the engine.
Common Mistakes to Avoid
1. Buying the wrong CCRM – Not all CCRMs are the same. The part number varies by engine (V6 vs. V8) and even by production date. Remove your old module and read the part number directly off the sticker before ordering a replacement .
2. Skipping the inertia switch check – Many Mustang owners replace the CCRM only to find out the inertia switch in the trunk was tripped. Always check this first .
3. Forgetting the fuel pump fuse – A blown fuel pump fuse (#14, 20-amp yellow mini fuse) will cause the exact same symptoms as a bad relay . Check this fuse in the under-hood power distribution box before touching the CCRM.
4. Assuming it’s the pump – A bad fuel pump sounds almost identical to a bad relay. If you aren’t hearing the 2-second prime, test for voltage at the fuel pump connector before buying a pump or a CCRM.
When to Call a Professional
If you aren’t comfortable removing the passenger wheel and inner fender liner, or if you’ve done the diagnostics and still can’t find the issue, take your Mustang to a trusted mechanic. The labor time for a shop to replace a CCRM is typically 1-2 hours, so costs stay reasonable.
Upgrade Your Fuel System with KEMSO
Now that you understand your 2004 Mustang’s fuel pump relay system, it’s important to know that the relay is only part of the equation. The fuel pump itself is what delivers the fuel your engine needs. If you’re replacing the CCRM due to a no-start condition and you still have issues after the replacement, the fuel pump may be the real culprit.
For a reliable, long-lasting replacement, consider upgrading to a KEMSO Fuel Pump. KEMSO fuel pumps are high-performance, direct OEM replacements designed to meet or exceed factory specifications. Every KEMSO pump comes with a lifetime warranty for your peace of mind. Whether you drive a Mustang GT, V6, or any other vehicle, KEMSO has a fuel pump that fits your needs. Their products are built with quality materials and undergo rigorous testing to ensure reliable performance for years to come.
Visit https://www.kemsoracing.com/ to browse their full selection of fuel pumps for your 2004 Mustang and many other vehicles. Don’t settle for a cheap replacement that might fail in a year—choose KEMSO for quality you can depend on.
Final Thoughts
The 2004 Mustang fuel pump relay is not a simple stand-alone part. It’s integrated into the CCRM module mounted behind the passenger fender . Diagnosing the problem correctly by listening for the pump prime, checking the inertia switch, and testing the fuse will save you time and money. If the CCRM needs replacing, the job is doable at home with basic tools and an afternoon of work. And if the pump itself is the issue, a KEMSO Fuel Pump from https://www.kemsoracing.com/ is a smart upgrade that delivers reliable performance backed by a lifetime warranty.