International · SCR NOx Conversion Efficiency MonitorMedium

International SPN 4364 FMI 15: SCR NOx Conversion Efficiency Below Normal

SPN 4364 FMI 15 (FMI 15) — Aftertreatment 1 SCR Catalyst NOx Conversion Efficiency - Data Valid But Above Normal Operating Range - Least Severe Level

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Quick Answer

SPN 4364 FMI 15 means SCR NOx conversion efficiency is trending below target at the least-severe warning level. It's a medium-severity code with no derate yet - check DEF quality and doser spray pattern before it escalates.

Overview: International SPN 4364 FMI 15

This is the earliest stage of an SCR efficiency warning, giving you the best chance to fix a cheap DEF quality issue before it develops into a derate-triggering fault. ProfessionalDieselRepair.com always starts with a refractometer check on DEF quality, since it's the fastest and cheapest thing to rule out.

Repair Snapshot

Time

1 - 2.5 Hours

Difficulty

Intermediate

Parts Cost

$0 - $400

Dealer Labor

$350+

Tools Required

  • Jaltest Diagnostic Tool
  • Diesel Laptops Scan Tool
  • DEF Refractometer

What SPN 4364 FMI 15 Means

SPN 4364 FMI 15 is the earliest warning stage that SCR NOx conversion efficiency is trending below target. On International A26 engines, this is a genuine emissions performance signal, but at the least-severe FMI level there's no derate yet - it's a chance to correct DEF quality or dosing before the fault escalates.

Symptoms Drivers Notice

  • Amber check engine lamp, no derate at this stage
  • No noticeable drivability complaint
  • Possible slightly increased DEF consumption

Most Likely Root Causes

Diluted or contaminated DEF fluid

Common

DEF that doesn't meet the 32.5% urea concentration specification reduces NOx conversion effectiveness even with a perfectly functioning doser.

Partially clogged or worn doser injector

Occasional

An inconsistent spray pattern from the doser reduces even DEF distribution across the catalyst, lowering measured conversion efficiency.

Aging or degraded NOx sensors

Occasional

Inlet or outlet NOx sensors drifting out of calibration can report an efficiency reading that doesn't reflect the catalyst's true performance.

Common Misdiagnoses (Don't Waste Parts)

Replacing NOx sensors before checking DEF quality

Why it happens: A parts-cannon habit of assuming sensor failure on any efficiency-related code, skipping the far cheaper and more common DEF quality check.

Do this instead: Test DEF concentration with a refractometer first - contaminated or diluted DEF is a common and inexpensive fix compared to sensor replacement.

Master Tech Commentary

Mechanic's Notes

Shop tip: ProfessionalDieselRepair.com always tests DEF quality first on this code - low-quality or diluted DEF is a cheap, frequent cause that gets missed when techs jump straight to NOx sensor replacement.

Step-by-Step Diagnostic Procedure

  1. Step 1: Pull the fault on Navistar diagnostic software or Jaltest and check inlet versus outlet NOx sensor readings side by side.
  2. Step 2: Test DEF quality with a refractometer to confirm it meets the 32.5% urea concentration specification.
  3. Step 3: Inspect the DEF doser injector spray pattern for partial clogging or an inconsistent spray cone.
  4. Step 4: Check for any DEF crystallization buildup in the SCR mixing chamber, which can reduce effective dosing even with a healthy doser.
  5. Step 5: Run a parked regeneration if soot loading upstream may be affecting SCR inlet temperature and dosing enable conditions.
  6. Step 6: Clear the code and monitor NOx conversion efficiency trend data over the following drive cycles to confirm improvement.

Live Data & Freeze-Frame Tips

Compare inlet and outlet NOx sensor readings on Jaltest during steady highway cruise - a healthy system shows a significant drop from inlet to outlet; a smaller-than-expected drop confirms reduced conversion efficiency.

Safety Warnings

  • Wear gloves when handling DEF and testing with a refractometer.
  • Allow the exhaust system to cool before inspecting the doser injector.

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

  • Use only ISO 22241-certified DEF from a sealed, reputable source.
  • Inspect the doser injector spray pattern at scheduled aftertreatment service intervals.
  • Avoid storing DEF in direct sunlight or extreme temperatures, which can degrade its quality over time.

Related Fault Codes

SPN 4364 FMI 18

SCR NOx conversion efficiency below normal, moderately severe - the escalated version of this same fault if left unaddressed.

SPN 3363 FMI 1

DEF pump pressure below normal - a related dosing system fault that can indirectly contribute to reduced conversion efficiency.

Frequently Asked Questions

Can I drive with this code active?

Yes, there's no derate at this stage. Address it soon by checking DEF quality to prevent it from escalating into a more severe fault.

How do I reset this code without a scan tool?

Use Jaltest or Diesel Laptops to clear the code after confirming DEF quality and doser function are within specification.

What triggers the derate countdown on this code?

This least-severe FMI level doesn't start a derate countdown; sustained poor efficiency can eventually escalate to a more severe FMI level that does trigger inducement.

Is bad DEF really a common cause?

Yes, diluted, contaminated, or expired DEF is one of the most common and cheapest-to-fix causes of reduced conversion efficiency.

Do I need to replace NOx sensors for this code?

Not usually as a first step - check DEF quality and doser spray pattern first, since sensor replacement should only follow confirmed sensor-level testing.

How often should DEF quality be checked?

Check it any time this code appears, and consider spot-checking with a refractometer whenever DEF is purchased from an unfamiliar source.