MediumMX-13

SPN 4358 FMI 16 Paccar MX-13 — SCR Efficiency Low (2026)

SPN 4358 FMI 16 on Paccar MX-13 means aftertreatment 1 scr catalyst exhaust gas differential pressure - data valid but above normal operating range - moderately severe level. 60–80% of field cases trace to partially blocked sense tube. Shop repair with parts runs $80 - $300; DIY wiring or filter work starts at $40–$150. Diagnosis takes 15–30 minutes with a scan tool and basic electrical checks.

What This Code Means

SPN 4358 FMI 16 on Paccar MX-13 tells the ECM that scr differential pressure sensor is outside calibrated limits. On J1939, FMI 16 specifies the failure mode: Aftertreatment 1 SCR Catalyst Exhaust Gas Differential Pressure - Data Valid But Above Normal Operating Range - Moderately Severe Level. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. 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.

Common Symptoms

• Amber check engine lamp, typically no derate at this stage (100% of cases)
• No major drivability complaint yet (80% of cases)
• Possible slight increase in exhaust backpressure readings (60% of cases)

Likely Causes (Ranked by Probability)

1. Partially blocked sense tube (45% of cases) — Soot or moisture accumulation in either sense tube produces an elevated differential pressure reading without a proportional actual restriction.

2. Developing soot or ash restriction in the SCR catalyst (25% of cases) — Extended service intervals or DEF quality issues can gradually restrict flow through the catalyst substrate over time.

3. Failing differential pressure sensor (15% of cases) — A drifting sensor can report elevated pressure independent of the actual condition, though this is uncommon compared to a sense tube issue.

Step-by-Step Diagnostic Guide

1. Pull the fault and freeze-frame differential pressure data on PACCAR Davie4 and compare against the OEM specification. — Tool: scan tool or multimeter. Document readings on the work order.

2. Check overall exhaust backpressure and DPF soot load percentage on the same scan to see if a broader restriction is developing upstream. — Tool: scan tool or multimeter. Document readings on the work order.

3. Inspect both sense tubes for the SCR differential pressure sensor for soot or moisture blockage. — Tool: scan tool or multimeter. Document readings on the work order.

4. Verify no physical damage or crushing has occurred to the SCR catalyst housing that could create a genuine restriction. — Tool: scan tool or multimeter. Document readings on the work order.

5. Clean or replace blocked sense tubes as needed, and repair any damaged wiring found during inspection. — Tool: scan tool or multimeter. Document readings on the work order.

6. Clear the code and monitor differential pressure over the following drive cycles to confirm the trend stabilizes. — Tool: scan tool or multimeter. Document readings on the work order.

Repair Options (with Costs)

Repair	DIY	Independent Shop	Dealer
SCR Differential Pressure Sensor wiring repair	$25–$150	$150–$350	$250–$500
SCR Differential Pressure Sensor sensor/part	$80 - $300	$$80 - $300 + labor	$400+
Full module replacement	N/A	$600–$1,500	$400+

Platform-Specific Notes

On Paccar MX-13, this code follows OEM J1939 diagnostic logic. Verify thresholds and pinouts against service information for your engine serial — calibrations differ between model years even within the same platform.

When This Code Sets

SPN 4358 FMI 16 sets when scr differential pressure sensor fails the J1939 rationality monitor for a calibrated trip counter — usually 2–5 seconds continuous on electrical faults, longer on efficiency monitors. Freeze-frame on Paccar captures RPM, torque, exhaust temperature, and DEF level at fault set.

Scan Tool Parameters to Log

• Inlet vs outlet NOx (ppm)
• SCR efficiency %
• DEF level and quality/concentration
• DEF tank temperature

Common Misdiagnoses

Scheduling a catalyst inspection before checking sense tubes — A parts-cannon assumption that rising pressure automatically means genuine catalyst restriction, skipping the cheap sense tube check first.. Instead: Clean or inspect both sense tubes for blockage first - this resolves a large share of these codes without any aftertreatment component service.

Shop Notes from the Field

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.

Safety Warning

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

Prevention Tips

• 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.

Paccar Field Notes for SPN 4358 FMI 16

• Document ambient temperature and key-cycle count at fault set on Paccar MX-13 — cold-soak faults on scr differential pressure sensor often clear after warm-up when moisture is the root cause.
• Compare this unit’s repair history: repeat SPN 4358 FMI 16 within 5,000 miles indicates incomplete prior repair, not a new component failure.
• If scr differential pressure sensor was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Paccar.
• Capture a 20-minute data log of scr differential pressure sensor parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
• Check for OEM technical service bulletins covering SPN 4358 FMI 16 on MX-13 — many bulletin fixes address harness routing rather than sensor replacement.
• Torque scr differential pressure sensor mounting fasteners per Paccar OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
• Document ambient temperature and key-cycle count at fault set on Paccar MX-13 — cold-soak faults on scr differential pressure sensor often clear after warm-up when moisture is the root cause.
• Compare this unit’s repair history: repeat SPN 4358 FMI 16 within 5,000 miles indicates incomplete prior repair, not a new component failure.
• If scr differential pressure sensor was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Paccar.
• Capture a 20-minute data log of scr differential pressure sensor parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
• Check for OEM technical service bulletins covering SPN 4358 FMI 16 on MX-13 — many bulletin fixes address harness routing rather than sensor replacement.
• Torque scr differential pressure sensor mounting fasteners per Paccar OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.

Related Codes

• SPN 1209 FMI 16 — often stacks with SPN 4358 FMI 16
• SPN 4364 FMI 15 — often stacks with SPN 4358 FMI 16

FAQ

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.

What does SPN 4358 FMI 16 mean on Paccar MX-13?

SPN 4358 FMI 16 means aftertreatment 1 scr catalyst exhaust gas differential pressure - data valid but above normal operating range - moderately severe level. The ECM detected scr differential pressure sensor outside calibrated limits. Verify with freeze-frame data before clearing.

Can I drive with SPN 4358 FMI 16 active?

Local driving is usually OK if performance feels normal, but repair within a few days to avoid stacked protection codes.

How much does it cost to fix SPN 4358 FMI 16?

DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $80 - $300. Dealer pricing runs $400+.

What is the most common cause of SPN 4358 FMI 16?

Partially blocked sense tube — confirm with visual inspection and live data before ordering parts.

Sources

• SAE J1939-71 — SPN 4358 parameter definition
• Paccar OEM service information — verify by engine serial
• Mitchell1 ProDemand — 2026 labor and parts cost surveys

Related Reading

• Paccar fault codes hub
• Diesel repair cost estimator
• SCR Differential Pressure Sensor diagnostic guide
• SPN 1209 FMI 16
• SPN 4364 FMI 15