SPN 3719 FMI 31: DPF Differential Pressure - Signal Erratic

What does SPN 3719 FMI 31 mean?

SPN 3719 FMI 31 is commonly tied to **DPF (Diesel Particulate Filter) differential pressure** logic. In plain English: the ECM thinks the pressure drop across the DPF system is **not believable**. “FMI 31” is typically a “condition exists” / “out of range” style complaint depending on OEM calibration. On many heavy-duty platforms, this shows up when the ECM sees **erratic, implausible, or inconsistent delta-P behavior** compared to what it expects for engine load and exhaust flow.

The ECM uses DPF differential pressure (delta-P) to estimate **soot loading**, regen effectiveness, and whether the exhaust path is restricted. If the sensor signal jumps around, reads negative, reads zero when it should not, or spikes unrealistically, the ECM cannot trust soot estimates. That can trigger warning messages, regen problems, and eventually derate or inducement depending on what other aftertreatment faults are present.

This fault can be caused by a genuinely restricted DPF, but very often it is caused by the boring stuff nobody wants to check: **plugged sensor tubes, bad hose routing, water in the lines, or wiring damage**.

Can I drive with SPN 3719 FMI 31?

Usually yes for a short distance, but treat it seriously because it can turn into downtime. Here’s why: delta-P is one of the inputs the ECM uses to decide whether regens are needed and whether the aftertreatment is behaving. If the delta-P signal is truly erratic, the ECM may:

• miscalculate soot load,
• schedule regens too often (or refuse them),
• or flag the system as unreliable and begin protection strategies.

If you also have regen inhibited faults, soot load faults, SCR/NOx faults, or active derate warnings, do not plan a long trip until you confirm what is actually happening.

How the system works (why this code happens)

A differential pressure sensor measures pressure upstream and downstream of the DPF (or across a reference point in the exhaust). Under normal conditions:

• At idle: delta-P is low and stable.
• Under load: delta-P rises smoothly with exhaust flow.
• During regen: behavior changes but it should still be physically believable.

The ECM expects delta-P to behave in a predictable way relative to:

• engine speed and load,
• exhaust flow,
• EGR state (platform-dependent),
• and known sensor operating ranges.

SPN 3719 FMI 31 appears when the ECM sees delta-P that does not match physics. “Erratic” usually means one of three things:

1) The sensor is actually receiving unstable pressure signals (blocked or damaged tubes cause pulsing or delay).

2) The electrical signal is unstable (wiring, connector, ground, power, internal sensor fault).

3) The exhaust system is leaking or altered in a way that ruins the measurement.

Location hint: where to look

On most trucks, the **DPF differential pressure sensor** is mounted on a bracket near the firewall, frame rail, or aftertreatment assembly. It will have:

• an electrical connector, and
• two small pressure hoses/tubes going to fittings on the exhaust (often one pre-DPF, one post-DPF).

Common real-world failure points:

• hoses routed too close to heat shields (melted or brittle),
• soot packing in the tubes at the exhaust nipples,
• water/condensation collecting in low spots,
• tubes kinked after service work,
• fittings partially blocked by carbon.

If you see soot trails, oily grime, or “it looks cooked,” it probably is.

Step-by-step diagnostic (safe, practical, repeatable)

Step 1: Confirm whether the fault is ACTIVE

Use a scan tool and answer two questions:

• Is SPN 3719 FMI 31 active right now?
• What other codes are active at the same time?

Stored-only codes are historical. Active codes are where diagnosis is easiest.

Step 2: Visual inspection (fast wins)

With the engine off and cool enough to work around:

• Inspect the DP sensor connector for corrosion, loose pins, or oil/DEF contamination.
• Follow the harness and look for rub-through near frame brackets, clamps, and heat shields.
• Inspect both pressure lines end-to-end:
• kinks,
• cracks,
• melted sections,
• loose fittings,
• and any low spots that could trap moisture.

If the lines are rubber, squeeze them gently. If they feel brittle or “crunchy,” replace them. If they feel soft and swollen, replace them. Humans always want a magical scan tool fix. Rubber hoses do not care about your feelings.

Step 3: Check for soot blockage and water intrusion

This is a big one. Soot and condensation can turn those tiny tubes into a science experiment.

• Remove the lines carefully.
• Check the exhaust nipples for blockage.
• Blow through the lines (low pressure). If you can’t blow through easily, they are restricted.
• If water dumps out of a line, you found a problem. Drain, clean, and reroute so it does not become a water trap again.

A restricted or waterlogged tube will cause delayed response and spikes. The ECM sees that as implausible delta-P behavior.

Step 4: Compare delta-P readings at idle and under load

With the scan tool:

• Watch delta-P at warm idle. It should be low and steady.
• Increase RPM gradually. Delta-P should rise smoothly.
• If the reading jumps, drops to zero randomly, or goes negative, that points strongly to an electrical/sensor problem or a line issue.

If possible, compare with a known-good truck or service spec ranges for your engine family.

Step 5: Electrical sanity checks (if the signal looks “digital chaos”)

If you suspect wiring:

• Confirm sensor power and ground (multimeter).
• Inspect pin fitment. “Looks connected” is not the same as “makes electrical contact.”
• Wiggle test the harness with the engine running while watching sensor readings. If the value jumps, you likely have harness damage.

Step 6: Check for exhaust leaks

Leaks upstream or near the pressure ports can ruin measurement:

• Look for soot trails around clamps, joints, and bellows.
• Listen for leaks under load (sometimes easier with a helper).
• A leak can cause delta-P to look too low or unstable.

Step 7: If everything checks out, suspect the sensor itself

If hoses are clean and routed correctly, fittings are clear, wiring is clean, and the signal still behaves badly, replace the differential pressure sensor with a quality part.

After replacement:

• clear faults,
• road test,
• confirm the code stays inactive,
• verify regen behavior if your tool supports it.

Decision guide (fast path)

• If the fault is STORED only: inspect hoses/tubes and wiring, clear codes, and monitor.
• If ACTIVE and you see obvious hose damage or soot blockage: fix that first, then retest.
• If ACTIVE and hoses are clean but the reading is still jumping: focus on wiring/connector and sensor replacement.
• If you also have soot load / regen inhibited / backpressure faults: evaluate whether the DPF is actually restricted and whether regen is completing successfully.

Common misdiagnosis mistakes (how people waste money)

1) Replacing the sensor immediately without checking the pressure tubes. Plugged tubes make a brand-new sensor look “bad.”

2) Ignoring water intrusion. Condensation in lines causes erratic readings and repeat faults.

3) Assuming “DPF is clogged” without confirming delta-P behavior under load and checking regen history.

4) Clearing the code repeatedly and hoping it stops coming back. That is not a repair plan. That is denial with extra steps.

Will SPN 3719 FMI 31 cause derate?

Sometimes. On many platforms, this fault starts as a warning. But if the ECM cannot trust delta-P and soot estimation, it may:

• limit regen requests,
• flag soot calculations as unreliable,
• or stack with other aftertreatment codes that trigger derate.

Derate risk goes up fast when SPN 3719 is accompanied by:

• regen failure or regen inhibited codes,
• high soot load estimates,
• exhaust temperature sensor faults,
• or air/fuel issues that drive soot production.

“Do this first” checklist (the stuff that fixes most trucks)

1) Confirm ACTIVE vs stored.

2) Inspect the DP sensor connector and harness routing for rub-through and corrosion.

3) Remove and inspect both DP sensor tubes/hoses for soot blockage, cracks, melting, and water intrusion.

4) Check delta-P behavior at idle and during a controlled RPM increase. Look for smooth, believable changes.

5) Fix exhaust leaks near the ports/clamps if found.

6) If the signal remains erratic after fixing tubes and wiring, replace the DP sensor and verify with a road test.

Prevention tips (so you don’t see it again next week)

• During service, inspect and clean DP sensor port fittings before they fully plug.
• Avoid hose routing that creates low spots where water pools.
• Secure the harness away from heat shields and sharp brackets.
• Fix small exhaust leaks early. They snowball into sensor logic problems.
• Address high soot conditions (boost leaks, EGR issues, injector problems) that overload the DPF and accelerate tube clogging.

Quick reference (for technicians in a hurry)

**Most common root cause:** blocked or waterlogged DP sensor tubes/hoses.

**Most common “it came back” reason:** routing not corrected, so moisture/soot re-accumulates.

**Best first test:** compare delta-P at idle vs a steady raised RPM while watching for spikes or dropouts.

If the code repeats after clearing, treat it as ACTIVE and diagnose it properly. It is cheaper than a tow.