Seeing the SPN 3720 FMI 15 warning on your screen can look serious, but don’t worry — it’s usually not a big problem. This code simply means your machine’s Diesel Particulate Filter is getting full of soot and needs to be cleaned. The cleaning process is called regeneration.

But if you ignore this early warning, the DPF will get more clogged. That can cause your machine to lose power or even stop running until it’s fixed. This guide will explain what the code means, why it happens, and how to fix it step by step.

What Exactly Is SPN 3720 FMI 15?

This code means:

Aftertreatment 1 Diesel Particulate Filter Differential Pressure — Data Valid but Above Normal (Least Severe Level).

Let’s make that simple:

1. Your machine checks pressure in the exhaust system before and after the DPF.
2. If the difference gets higher than normal, it means the DPF is starting to fill with soot.
3. FMI 15 means the reading is real, but not very severe yet. It’s an early warning that your system needs cleaning.

In plain words, your machine is saying:

“The DPF is starting to get full. Please run regeneration soon.”

What Are the Common Symptoms?

When this code appears, you’ll likely notice these signs:

1. DPF Warning Light: A small DPF symbol lights up or flashes on the dashboard.
2. Message on the Screen: It may say something like “Regen Required.”
3. No Power Loss Yet: The machine still runs normally at this stage.
4. HEST Light On: The “High Exhaust Temperature” light may come on during regeneration.

Why Does SPN 3720 FMI 15 Happen? The 4 Most Common Causes

This code doesn’t usually mean something is broken — it’s often part of normal operation.

Here are the most common reasons it appears:

1. Normal Soot Build-Up (Most Common)

Every diesel engine makes soot. The DPF’s job is to catch it.

After hours of work, soot naturally builds up. The code reminds you it’s time to clean the filter through regeneration.

2. Incomplete or Blocked Regeneration Cycles

Sometimes the regen process doesn’t start or finish.

This can happen if:

1. The “Regen Inhibit” switch is turned on (to prevent regen near flammable areas).
2. The engine isn’t hot enough or shuts off too soon.
3. You keep running the machine at light loads or short intervals.

3. Regeneration System Not Working

The machine can only clean itself if sensors and injectors are working correctly.

If another fault (like a bad temperature sensor or clogged fuel doser) exists, the system can’t complete regeneration — soot keeps building up.

4. Engine Produces Too Much Soot

If this code shows up much more often than normal, the engine might have its own problem.

Common causes include:

1. Leaking fuel injectors
2. Turbocharger trouble
3. EGR (exhaust gas recirculation) system faults

Tip: Think of FMI 15 as your machine saying, “Please clean my filter.” It’s not a breakdown — just a reminder to do regular maintenance.

How to Fix SPN 3720 FMI 15: A 4-Step Action Plan

Follow these steps one by one. In most cases, Step 1 will solve the problem.

Step 1: Perform a Parked (Forced) Regeneration

This is the main fix for FMI 15.

1. Find a Safe Spot: Park the machine on level ground, away from anything that can catch fire. The exhaust will get very hot.
2. Check the Manual: Find how your specific machine starts a parked regen — it’s often a button or switch.
3. Start the Regen: Hold the regen button or switch for several seconds. The engine RPM will rise. The process takes about 20–60 minutes.
4. Let It Finish: Do not touch anything until it’s done. When complete, the DPF light and code should clear.

In most cases, this one step fixes the issue.

Step 2: Check for Regen Inhibitors

If you can’t start a regen, make sure everything is set correctly:

1. The “Regen Inhibit” switch is OFF.
2. The parking brake is engaged.
3. The transmission is in neutral or park.
4. PTO (power take-off) is turned off.

Missing one of these can stop regen from starting.

Step 3: Inspect the Regeneration System

If the regen won’t start or doesn’t finish, check the supporting parts:

1. Look for other fault codes related to exhaust sensors or the fuel doser (a small injector that creates heat during regen).
2. Inspect the doser injector — it can get clogged with carbon and needs cleaning.
3. Fix any temperature sensor issues before retrying regeneration.

Step 4: Get a Professional DPF Cleaning

If you’ve tried regeneration a few times and the code keeps coming back, the DPF might be full of ash (not soot), which can be one of the main symptoms of a clogged DPF. Ash can’t burn off — it must be cleaned by a service shop. A technician can remove the DPF and clean it using a “bake and blow” process. If cleaning doesn’t work, the filter might need replacing.

When Should You Replace the DPF Pressure Sensor?

Rarely for this code.

SPN 3720 FMI 15 means the sensor is working correctly and giving a true reading — the filter really is restricted.

Replacing the sensor won’t fix a blocked DPF.

Only replace it if you have other J1939 Fault Codes showing the sensor itself has failed (for example, FMI 3 or FMI 4).

Conclusion

SPN 3720 FMI 15 is a normal, early warning — not an emergency. It means your Diesel Particulate Filter (DPF) is full of soot and needs cleaning.

1. Run a parked regeneration.
2. Check for switches or sensors blocking it.
3. If needed, clean or repair the regen system.
4. If it still won’t clear, have the DPF professionally cleaned.

Handle this code right away to avoid power loss or costly downtime. With regular regeneration and filter maintenance, your machine will keep running strong and clean for a long time. MechLink offers compatible filters to cater to your needs. Shop now!