Paccar · SCR Differential Pressure SensorMedium

PACCAR SPN 4358 FMI 16: SCR Differential Pressure Above Normal

SPN 4358 FMI 16 (FMI 16) — Aftertreatment 1 SCR Catalyst Exhaust Gas Differential Pressure - Data Valid But Above Normal Operating Range - Moderately Severe Level

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Quick Answer

SPN 4358 FMI 16 means the pressure drop across the SCR catalyst is above normal on your PACCAR MX engine. It's a medium-severity early warning with no derate - check the sense tubes for blockage before assuming a genuine catalyst restriction.

Overview: Paccar SPN 4358 FMI 16

This code is one of the aftertreatment system's earlier warning stages, giving you time to investigate before an actual derate-triggering backpressure fault develops. ProfessionalDieselRepair.com checks the sense tubes first since a partial blockage there is far more common than genuine catalyst restriction.

Repair Snapshot

Time

1 - 2 Hours

Difficulty

Intermediate

Parts Cost

$80 - $300

Dealer Labor

$400+

Tools Required

  • PACCAR Davie4
  • Texa Diagnostic Tool
  • Multimeter

What SPN 4358 FMI 16 Means

SPN 4358 FMI 16 means the pressure drop across the SCR catalyst is running above normal - a sign of a developing restriction in that section of the aftertreatment system. On PACCAR MX platforms, this is an early warning that's worth investigating before it develops into a broader exhaust backpressure or derate-triggering fault.

Symptoms Drivers Notice

  • Amber check engine lamp, typically no derate at this stage
  • No major drivability complaint yet
  • Possible slight increase in exhaust backpressure readings

Most Likely Root Causes

Partially blocked sense tube

Common

Soot or moisture accumulation in either sense tube produces an elevated differential pressure reading without a proportional actual restriction.

Developing soot or ash restriction in the SCR catalyst

Occasional

Extended service intervals or DEF quality issues can gradually restrict flow through the catalyst substrate over time.

Failing differential pressure sensor

Rare

A drifting sensor can report elevated pressure independent of the actual condition, though this is uncommon compared to a sense tube issue.

Common Misdiagnoses (Don't Waste Parts)

Scheduling a catalyst inspection before checking sense tubes

Why it happens: A parts-cannon assumption that rising pressure automatically means genuine catalyst restriction, skipping the cheap sense tube check first.

Do this instead: Clean or inspect both sense tubes for blockage first - this resolves a large share of these codes without any aftertreatment component service.

Master Tech Commentary

Mechanic's Notes

Shop tip: ProfessionalDieselRepair.com always checks both sense tubes for blockage before assuming a genuine SCR restriction on this code - it's a cheap, fast check that avoids an unnecessary catalyst inspection appointment.

Step-by-Step Diagnostic Procedure

  1. Step 1: Pull the fault and freeze-frame differential pressure data on PACCAR Davie4 and compare against the OEM specification.
  2. Step 2: Check overall exhaust backpressure and DPF soot load percentage on the same scan to see if a broader restriction is developing upstream.
  3. Step 3: Inspect both sense tubes for the SCR differential pressure sensor for soot or moisture blockage.
  4. Step 4: Verify no physical damage or crushing has occurred to the SCR catalyst housing that could create a genuine restriction.
  5. Step 5: Clean or replace blocked sense tubes as needed, and repair any damaged wiring found during inspection.
  6. Step 6: Clear the code and monitor differential pressure over the following drive cycles to confirm the trend stabilizes.

Live Data & Freeze-Frame Tips

Track SCR differential pressure alongside overall exhaust backpressure on Davie4 during a drive cycle - if backpressure stays normal while only this specific reading is elevated, suspect a sense tube issue rather than genuine catalyst restriction.

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 - it's an early warning stage with no active derate, safe to address at the next scheduled service.

Prevention Tips for Fleets & Owner-Operators

  • Inspect sense tubes for soot and moisture buildup during scheduled aftertreatment service.
  • Use only ISO 22241-certified DEF to avoid dosing issues that can contribute to catalyst restriction.
  • Address any related dosing or NOx sensor faults promptly.

Related Fault Codes

SPN 1209 FMI 16

Overall exhaust backpressure above normal - a broader measurement that can confirm whether this reading reflects a system-wide restriction.

SPN 4364 FMI 15

SCR NOx conversion efficiency below normal - a related fault that can develop alongside genuine catalyst restriction issues.

Frequently Asked Questions

Can I drive with this code active?

Yes, there's no derate at this stage. It's a good idea to have it checked soon, but it's safe to drive in the meantime.

How do I reset this code without a scan tool?

Use PACCAR Davie4 or Texa to clear the code after confirming the sense tubes are clear or any restriction is resolved.

What triggers the derate countdown from this code?

This specific code doesn't start a derate countdown; it's an early warning that can escalate into a more severe backpressure fault if the underlying cause isn't addressed.

Is this always a sign of catalyst damage?

No, a blocked sense tube is a more common and far cheaper explanation - check that before assuming the catalyst itself needs service.

Will this affect my emissions compliance?

Not directly at this stage, but a genuine developing restriction should be addressed before it worsens into a performance-affecting fault.

How often should this be checked?

It's continuously monitored by the ECM, but reviewing the trend at each service interval helps catch a developing restriction early.