How to Fix SPN 4364 FMI 18 – NOx Sensor Drift / SCR Efficiency Low

How to Fix SPN 4364 FMI 18 – NOx Sensor Drift / SCR Efficiency Low

If your truck is showing SPN 4364 FMI 18, your SCR system is failing its efficiency test, often due to inaccurate readings from the NOx sensors.

This is one of the top 3 most-searched aftertreatment codes because it leads to:

  • Forced parked regens

  • Failed regens

  • DEF derates (SPN 5246)

  • Final inducement 5 MPH derate (SPN 3712)

This code affects:

  • Cummins ISX / X15

  • Detroit DD13/DD15

  • Paccar MX-13 / MX-11

  • Volvo/Mack

  • International A26

  • Freightliner Cascadia / Kenworth / Peterbilt

  • 📞 Call: (800)-442-1752
    👉 Shop DieselScanners Diagnostic Kits

🚨 What SPN 4364 FMI 18 Means

The engine control module has determined:

✔ The SCR catalyst is not reducing NOx properly

✔ NOx outlet values are too similar to inlet values

✔ The NOx sensor reading is drifting or incorrect

✔ SCR efficiency appears below acceptable limits

This code often triggers:

  • CEL (Check Engine Light)

  • MIL (Malfunction Indicator Lamp)

  • DEF derate countdown

  • Limited power mode

⚠️ Common Causes of SPN 4364 FMI 18

This fault is almost always caused by one of the following:

🔹 1. Bad NOx Sensor (MOST COMMON)

NOx outlet sensors drift over time due to heat cycling.

🔹 2. DEF Quality Problems

If DEF concentration is off → SCR reduction fails.

🔹 3. Failed DEF Doser / Pump

If DEF does not dose properly, NOx fails to drop.

🔹 4. SCR Catalyst Degradation

The catalyst becomes contaminated or worn out.

🔹 5. Exhaust Leaks Before SCR

Leaking pipes → incorrect NOx readings.

🔹 6. Bad Temperature Sensors

Incorrect temp prevents SCR from activating properly.

🔹 7. Software Updates Required

Especially on Paccar & Detroit engines.

🔧 How to Diagnose SPN 4364 FMI 18 Using a Truck Scanner

A professional truck scanner is REQUIRED to diagnose this code.
You need access to:

  • NOx readings

  • SCR efficiency tests

  • DEF dosing commands

  • Temperature verification

  • Aftertreatment resets

1️⃣ Compare Inlet vs Outlet NOx Sensor Readings

During normal operation:

  • Outlet NOx should be much lower than inlet

  • If readings are too close → SCR is failing

Example:

  • Inlet: 450 ppm

  • Outlet: 420 ppm (bad)

  • Outlet: 80–200 ppm (good)

2️⃣ Run NOx Sensor Functional Tests

Your scanner should:

✔ Command NOx heater tests
✔ Check communication
✔ Check response time
✔ Verify ppm values

If sensor responds slowly → replace it.

3️⃣ Perform DEF Doser Quantity Test

Look for:

  • Proper spray pattern

  • Correct flow rate

  • No crystallization

  • No clogging

Failed doser = SCR inefficiency.

4️⃣ Run SCR Efficiency Test

This test determines:

✔ Is the SCR catalyst still functional?
✔ Is NOx reduction within spec?
✔ Is a derate imminent?

If SCR fails → catalyst may need replacement.

5️⃣ Test DEF Quality

Use the scanner to run:

  • DEF concentration test

  • DEF quality test

If DEF is below 31.8% urea → contamination detected.

6️⃣ Check Aftertreatment Temperatures

Verify:

  • DOC inlet/outlet

  • DPF inlet/outlet

  • SCR inlet temps

Incorrect temps = failed SCR reaction.

🛠 How to Fix SPN 4364 FMI 18 (By Root Cause)

✔ If Outlet NOx Sensor is Failing

➡ Replace sensor
➡ Run NOx sensor reset
➡ Perform SCR efficiency test
➡ Clear derate

(Most common fix)

✔ If DEF Quality is Bad

➡ Drain DEF tank
➡ Refill with fresh DEF
➡ Run DEF quality test
➡ Reset aftertreatment system

✔ If DEF Pump or Doser Fails Tests

➡ Replace pump or doser
➡ Prime DEF system
➡ Run DEF dosing test

✔ If SCR Catalyst is Worn Out

➡ Replace SCR
➡ Run SCR adaptation reset
➡ Perform a forced regen

✔ If Temperature Sensors Are Incorrect

➡ Replace faulty sensor
➡ Clear code
➡ Repeat SCR performance test

✔ If Exhaust Leak is Found

➡ Repair leak
➡ Run SCR test to verify fix

🚫 MYTHS About SPN 4364

❌ “You can clear the code and keep driving.”

No — this code always returns.

❌ “Adding more DEF fixes it.”

DEF quality is NOT the only factor.

❌ “Unplugging NOx sensors resets them.”

They must be recalibrated with a scanner.

❌ “You can bypass SCR problems.”

Not legally — and derates WILL come.

⭐ The Only Real Way to Fix SPN 4364: Use a Professional Truck Scanner

To actually fix this code, your scanner must:

✔ Read and graph NOx values
✔ Run SCR efficiency tests
✔ Run DEF dosing tests
✔ Command NOx resets
✔ Reset inducement countdown
✔ Clear SCR faults after repair

Dealers charge $450–$1,200 for this diagnosis.

You can do it yourself with:

🚀 DieselScanners™ Laptop Diagnostic Kit

The #1 diagnostic kit for SCR, DEF, and NOx issues.

Why truckers choose DieselScanners:

✔ OEM-level software
✔ NOx sensor resets
✔ SCR efficiency testing
✔ DEF dosing commands
✔ Forced regen capability
✔ Works on all major diesel engines
✔ Laptop arrives activated & ready
✔ Zero subscription fees

👉 Fix SPN 4364 today at DieselScanners.com

📌 Final Takeaway

SPN 4364 FMI 18 means your SCR system is not reducing NOx properly.
This is usually caused by:

  • Bad NOx sensors

  • Bad DEF

  • DEF pump/doser issues

  • SCR catalyst degradation

  • Incorrect aftertreatment temps

With a professional truck scanner, you can diagnose the exact issue, repair it correctly, reset the system, and avoid a 5 MPH derate.

📞 Call: (800)-442-1752
👉 Shop DieselScanners Diagnostic Kits

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