Caterpillar · SCR Differential Pressure SensorMedium

Caterpillar SPN 4358 FMI 2: SCR Differential Pressure Signal Erratic

SPN 4358 FMI 2 (FMI 2) — Aftertreatment 1 SCR Catalyst Exhaust Gas Differential Pressure - Data Erratic, Intermittent, or Incorrect

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Quick Answer

SPN 4358 FMI 2 means the SCR differential pressure sensor is sending an inconsistent signal. It's a medium-severity code with no derate on its own - check the sense tubes for blockage before replacing the sensor.

Overview: Caterpillar SPN 4358 FMI 2

This sensor is the aftertreatment system's early-warning gauge for SCR restriction, so an erratic signal leaves the ECM without a reliable way to catch a developing catalyst problem. ProfessionalDieselRepair.com always inspects both sense tubes for moisture or soot blockage first, since that's the more common cause of an intermittent signal than the sensor itself.

Repair Snapshot

Time

1 - 2 Hours

Difficulty

Intermediate

Parts Cost

$70 - $260

Dealer Labor

$350+

Tools Required

  • Cat ET (Electronic Technician)
  • Jaltest Diagnostic Tool
  • Diesel Laptops Scan Tool
  • Multimeter

What SPN 4358 FMI 2 Means

SPN 4358 FMI 2 means the SCR differential pressure sensor is sending an erratic or inconsistent signal to the ECM. This sensor helps the system detect SCR restriction or catalyst brick damage, so an unreliable signal doesn't cause immediate performance loss but does blind the aftertreatment monitoring system until it's fixed.

Symptoms Drivers Notice

  • Amber check engine lamp, typically no derate on its own
  • No immediately noticeable drivability change
  • Possible intermittent code setting and clearing on its own

Most Likely Root Causes

Blocked or partially clogged sense tube

Common

Soot or moisture accumulation in either sense tube produces an inconsistent pressure differential reading at the sensor.

Intermittent connector or wiring fault

Occasional

A loose or corroded connector pin can cause the signal to drop in and out under vibration.

Failing sensor element

Occasional

An internally degrading sensor can produce erratic output even with clean sense tubes and a sound harness.

Common Misdiagnoses (Don't Waste Parts)

Replacing the SCR differential pressure sensor without checking the sense tubes

Why it happens: It's a quick parts-cannon swap that skips the less obvious step of clearing both tubes first.

Do this instead: Inspect and clean both sense tubes before replacing the sensor - a blocked tube produces the identical erratic signal as a failing sensor.

Master Tech Commentary

Mechanic's Notes

Shop tip: ProfessionalDieselRepair.com checks both sense tubes for moisture and soot blockage before ever touching the sensor itself on this code - an erratic signal is a classic sign of a partially blocked tube rather than an actually failed sensor element.

Step-by-Step Diagnostic Procedure

  1. Step 1: Pull the fault and freeze-frame differential pressure data on Cat ET to look for erratic spikes or dropouts rather than a steady out-of-range value.
  2. Step 2: Locate the SCR differential pressure sensor and its two sense tubes routed from before and after the SCR catalyst.
  3. Step 3: Inspect both sense tubes for kinks, disconnection, or soot/moisture blockage that can cause erratic readings.
  4. Step 4: Back-probe the sensor's signal, ground, and power wires with a multimeter while wiggling the harness to check for an intermittent connection.
  5. Step 5: Clean or replace blocked sense tubes and repair any damaged wiring found during inspection.
  6. Step 6: Clear the code and monitor live differential pressure data across a drive cycle to confirm the signal is now stable.

Live Data & Freeze-Frame Tips

Watch differential pressure on Cat ET during a steady-state drive - a healthy sensor tracks smoothly with exhaust flow rate, while an erratic sensor or blocked tube shows sudden spikes, dropouts, or a signal that doesn't correlate with engine load.

Safety Warnings

  • Allow the exhaust system to cool before removing or inspecting sense tubes.
  • Wear gloves when handling soot-contaminated components.

When to Limp In vs Tow

Towing is not necessary for this code alone since there's no associated derate or safety risk from driving with it active.

Prevention Tips for Fleets & Owner-Operators

  • Inspect sense tubes for soot and moisture buildup during scheduled aftertreatment service.
  • Keep connector pins clean and free of corrosion during routine PM work.
  • Address any exhaust leaks upstream that could introduce excess soot into the sense tubes.

Related Fault Codes

SPN 3609 FMI 2

DPF intake pressure erratic - a related aftertreatment pressure sensor that can share the same sense tube blockage issues.

SPN 4364 FMI 15

SCR NOx conversion efficiency below normal - a separate but related fault that can develop if genuine SCR restriction goes undetected due to this sensor issue.

Frequently Asked Questions

Can I drive with this code active?

Yes, there's no derate tied to this code. It's safe to drive while scheduling a repair.

How do I reset this code without a scan tool?

Use Cat ET, Jaltest, or a similar tool to clear the code after confirming and repairing the sense tubes or wiring issue.

What triggers a derate on this code?

This specific code doesn't trigger a derate directly, but it can mask a genuine SCR restriction problem that eventually sets its own more serious fault.

Is the sensor usually bad, or is it the tubes?

Blocked sense tubes are the more common cause of an erratic signal than an actually failed sensor element.

Will this affect my emissions performance?

Not directly, but it does reduce the system's ability to detect a developing SCR restriction, so it shouldn't be ignored long-term.

How can I tell if it's intermittent versus constant?

Review the freeze-frame and occurrence count data on Cat ET - a code that sets and clears repeatedly points to an intermittent connection or partial blockage rather than a fully failed sensor.