SPN 3719 FMI 31: DPF Differential Pressure - Signal Erratic

Overview (what SPN 3719 FMI 31 is telling you)

SPN 3719 FMI 31 is commonly tied to the **aftertreatment DPF soot load logic** and/or the **DPF differential pressure (delta-P) signal plausibility** depending on the OEM calibration. In plain terms, the ECM expected the DPF pressure behavior to make sense for the current operating conditions, and it didn’t.

That mismatch can come from two broad buckets:

• **A real exhaust flow restriction** (soot loading, plugged DPF, failed regen strategy, exhaust restriction, etc.).
• **A measurement problem** (DP sensor/tubes/wiring giving the ECM nonsense, intermittent, or jumpy data).

Your job is to figure out which one is happening **today**, not which one sounds scarier.

Can I drive?

This is typically a **Warning-level** situation. You can often drive a short distance, but you should treat it like a “don’t ignore me” fault. Aftertreatment systems love to escalate: repeated drive cycles with bad data can become **regen inhibits, soot load climbing, torque derate**, and on some platforms, inducement steps.

If the fault is **ACTIVE right now**, diagnose it. If it’s **stored only**, use it as a clue and look at “what was happening when it set”.

What “erratic signal” usually means in real life

FMI 31 often gets used when the ECM believes a “condition exists” or the data is implausible. In practical diagnostics, “erratic” almost always means one of these:

1) **Pressure lines are not telling the truth**

Soot plugging, condensation, cracked lines, melted sections, or kinked routing will cause delta-P to jump. A partially plugged tube can behave “normal” at idle and become insane under load.

2) **Electrical intermittency**

Loose pins, connector fretting, corrosion, or rubbed wiring near the aftertreatment can make the signal “blink” or spike. This is extremely common around heat shields and frame brackets because vibration and heat do their usual teamwork.

3) **Sensor drift/failure**

DP sensors can fail in ways that still produce “a number”, just not a believable one. If you see readings that jump around with no correlation to RPM/load, you are not looking at real exhaust physics.

How the DP sensor system works (why delta-P matters)

The DPF differential pressure sensor measures the pressure **before** and **after** the DPF and calculates the difference (delta-P). Under normal conditions:

• At **idle**, delta-P should be relatively low and stable.
• Under **load**, delta-P rises as exhaust flow rises.
• As soot load increases, delta-P rises **more than normal** for the same load.
• During a regen, you may see changing values as temperatures and flow conditions change, but it still should not look like a broken seismograph.

The ECM uses this signal (plus temperature sensors, soot model calculations, and regen history) to decide:

• whether soot loading is plausible,
• whether a regen is needed,
• whether a regen is working,
• whether protection/derate strategies should begin.

So if the DP signal is untrustworthy, the ECM is basically blind and will often respond with conservative protections.

Quick “do this first” checklist (10 minutes)

Before you replace anything (because humans love buying parts they don’t need):

1) **Confirm status: ACTIVE vs stored**

• ACTIVE means it’s happening now and repeatable.
• Stored means it happened earlier. Look at other codes, regen history, and whether it returns after clearing.

2) **Look at live data at idle**

• Is delta-P stable? Or does it jump?
• If it jumps at idle with no changes in RPM, suspect wiring/sensor.

3) **Quick physical inspection**

• Find the DP sensor and follow both pressure tubes.
• Look for cracked rubber ends, melted sections, soot packing, loose clamps, or obvious leaks.

4) **Check connector health**

• Corrosion, pushed pins, loose lock tabs, oil/DEF contamination.
• If you can wiggle the connector and watch live data spike, congratulations, you found your problem.

Step-by-step diagnostic (safe, practical, and not “parts cannon”)

Step 1: Visual inspection of tubes/hoses (most common failure point)

DP sensors rely on small pressure tubes. Common failure modes:

• **Plugged with soot**: delta-P reads too high, too low, or erratic.
• **Condensation/water**: can cause slow response, spikes, or nonsense readings.
• **Kinked routing**: becomes worse when the engine torques or the chassis flexes.
• **Heat damage**: melted tube sections near the aftertreatment.

Action:

• Remove tubes and inspect through them (light/air).
• Clean or replace as needed.
• Make sure routing is correct and protected from heat.

Step 2: Check for exhaust leaks (skews delta-P math)

Leaks before/after the DPF can distort what the sensor sees, especially under load.

Clues:

• Soot trails around clamps/joints.
• Audible leak under throttle.
• A delta-P curve that makes no physical sense relative to load.

Action:

• Repair leaks first. A perfect sensor can’t fix broken exhaust plumbing.

Step 3: Verify wiring integrity (the “erratic” classic)

Even if tubes are perfect, wiring can ruin everything.

Look for:

• Chafing near brackets
• Heat-baked insulation near shields
• Corrosion in connectors
• Poor ground points

Action:

• Inspect and repair routing.
• Clean/repair pins.
• Secure harness away from rubbing points.

Step 4: Sensor plausibility testing

If tubes and wiring check out, test whether the sensor behaves logically:

• At idle: should be stable.
• As RPM/load rises: delta-P rises smoothly.
• No random spikes with steady throttle.

If you have a scan tool that shows “before DPF pressure” and “after DPF pressure” separately, look for:

• One side stuck
• Both moving oddly
• Values that don’t change with load

If it’s implausible after all upstream checks, replace the sensor.

Step 5: Determine if you have a real restriction problem

If sensor data is believable and delta-P is genuinely high:

• Soot load may be high
• Regen may be failing or inhibited
• DPF may be restricted

Clues:

• Frequent regen requests
• Aborted regens
• High soot model values
• Companion codes for temps, doser, NOx, or regen inhibits

Action:

• Address regen inhibit causes.
• Perform OEM-approved regen procedures.
• If restriction remains, clean/replace DPF per service guidance.

Decision guide (fast path)

• If SPN 3719 FMI 31 is STORED only:
• Inspect tubes and connector anyway.
• Clear and monitor.
• If it returns quickly, treat as ACTIVE.

• If ACTIVE and delta-P is jumpy:
• Tubes/hoses + wiring/connector first.
• Then sensor.

• If ACTIVE and delta-P is consistently high under load:
• Confirm no exhaust leaks.
• Confirm regen is possible.
• Then investigate soot loading / restriction.

Common misdiagnosis mistakes (how people waste money)

• Replacing the sensor without inspecting plugged tubes.
• Ignoring exhaust leaks that skew readings.
• Clearing codes repeatedly without checking whether the fault is ACTIVE.
• Treating one fault in isolation when regen/aftertreatment faults usually travel in packs.

Prevention tips

• Inspect DP tubes during service intervals, especially if you operate in soot-heavy duty cycles (idling, stop-and-go, short runs).
• Fix exhaust leaks early.
• Avoid prolonged idling when possible. Aftertreatment hates that lifestyle.
• If you wash the chassis/engine bay, avoid blasting water into connectors and sensor lines.

What to record for real troubleshooting

If you want repeatable diagnosis (and fewer guesses), record:

• Whether the fault is ACTIVE
• Delta-P at idle
• Delta-P under a steady load (same speed/gear)
• Regen history (completed vs aborted)
• Companion faults (temps, dosing, NOx, soot model)

That data makes your next step obvious instead of emotional.