SPN 4364 FMI 18 commonly indicates that the engine or aftertreatment controller has measured SCR NOx conversion efficiency below the expected range. On Detroit GHG14 systems the published description is “SCR NOx Conversion Efficiency Very Low,” while International A26 documentation describes SCR conversion efficiency low. The code does not automatically prove that the outlet NOx sensor or SCR catalyst is defective.
Aftertreatment components and exhaust gases become extremely hot during a parked regeneration. Work outside or use approved exhaust extraction, set the parking brake, chock the wheels and remove combustible material from the exhaust area. Follow the exact OEM procedure for the engine serial number.
What Does SPN 4364 FMI 18 Mean?
| SPN | 4364 — SCR conversion-efficiency monitoring |
|---|---|
| FMI | 18 — data valid but below the normal operating range, moderately severe |
| Typical description | SCR NOx conversion efficiency low or very low |
| System | Selective Catalytic Reduction, DEF dosing and NOx-sensor feedback |
| Possible reaction | MIL or check-engine lamp; derate behavior depends on engine family and calibration |
The controller compares operating conditions and the readings from the inlet and outlet NOx sensors. When the measured reduction in NOx is lower than the calibrated threshold for long enough, it stores SPN 4364 FMI 18. Because the calculation depends on the complete air, fuel, DEF and SCR system, replacing a sensor without testing can be expensive and ineffective.
Common Causes of SPN 4364 FMI 18
- Contaminated, diluted, old or incorrectly concentrated DEF
- Restricted, leaking, crystallized or under-performing DEF dosing components
- Biased inlet or outlet NOx-sensor readings
- Exhaust leaks before, between or near the NOx sensors
- DEF deposits or poor mixing inside the decomposition tube or SCR assembly
- Air-management, EGR or fuel-system faults that change engine-out NOx
- Low exhaust temperature or an incomplete verification drive
- Incorrect or outdated controller calibration on certain engine families
- Reduced SCR catalyst performance after the other systems have been verified
Symptoms You May Notice
- Check-engine or MIL lamp
- Aftertreatment warning or inducement message
- Possible power derate depending on the engine and fault history
- Frequent regeneration requests
- Higher-than-expected outlet NOx readings
- The code returning after a regen or after a NOx sensor was replaced
Diagnostic Order: What to Check First
1. Save the complete fault-code report
Record all active and inactive codes, timestamps and available freeze-frame data before clearing anything. Air-management, EGR, fuel, temperature, DEF-pressure or NOx-sensor circuit faults can be the reason the efficiency monitor failed. OEM bulletins instruct technicians to diagnose relevant companion codes first.
2. Confirm the exact engine and emissions generation
Detroit, Cummins, PACCAR, Volvo/Mack and International may use the same SPN/FMI combination with different enabling conditions and verification routines. Identify the engine model, model year, ECM/ACM software level and aftertreatment configuration before using a test specification.
3. Inspect and test DEF quality
Look for diesel fuel, oil, coolant, dirt or water contamination. Check the DEF concentration with an appropriate refractometer and compare the result with the current manufacturer specification. Do not rely only on the fluid's appearance.
4. Inspect the DEF delivery system
Check the tank, pickup, lines, connectors, dosing valve and decomposition area for leaks, kinks, damage or crystallized deposits. Run the OEM DEF quantity, pressure or dosing self-test. Required quantities and temperatures vary, so use the specification for the exact engine rather than a number copied from another platform.
5. Evaluate both NOx sensors
Check for NOx-sensor circuit codes, damaged wiring, loose connectors and implausible readings. Compare inlet and outlet NOx values under the enabling conditions defined by the manufacturer. A sensor can be electrically online yet biased; however, a low-efficiency code alone is not proof that either sensor should be replaced.
6. Check exhaust integrity and upstream engine systems
Inspect for exhaust leaks and review boost, intake-manifold pressure, EGR, fuel and temperature data. These systems affect engine-out NOx and can make a healthy SCR system appear inefficient.
7. Verify the repair correctly
After the cause is repaired, follow the OEM verification routine. That may include an SCR self-check, parked regeneration or loaded road test. Do not force a regeneration when another active fault prohibits it or when the DPF/aftertreatment has not been inspected for a dangerous condition.
Why Replacing the NOx Sensor May Not Fix It
SPN 4364 FMI 18 is an efficiency result, not a simple open- or short-circuit code. A new outlet sensor cannot correct contaminated DEF, inadequate dosing, an exhaust leak, incorrect engine-out NOx or a deteriorated catalyst. Test the system in sequence and use live data before ordering parts.
Diagnostic Software for SPN 4364 FMI 18
- Detroit DiagnosticLink (DDDL) for Detroit and Freightliner service routines, live data and fault troubleshooting
- Navistar Engine Diagnostics (NED) for supported International engine and aftertreatment systems
- JPRO for all-makes fault-code triage and live-data review
An all-makes tool is useful for identifying the system and companion codes, but an OEM application is normally required for detailed dosing tests, controller routines and manufacturer-specific verification.
Frequently Asked Questions
Can I keep driving with SPN 4364 FMI 18?
Some vehicles initially show only a warning lamp, while others progress toward an inducement or derate. Continuing to operate without diagnosing the cause can make the repair more expensive. Follow the instructions displayed by the vehicle and the OEM service information.
Does this code always mean the SCR catalyst is bad?
No. DEF quality, dosing, NOx-sensor bias, exhaust leaks and upstream engine faults must be tested first. Catalyst replacement should be supported by the OEM diagnostic procedure and test results.
Will a parked regen clear SPN 4364 FMI 18?
A regen may be part of the verification routine, but it does not repair contaminated DEF, failed dosing, sensor bias or a damaged catalyst. Correct active faults before attempting a regeneration.
Is SPN 4364 FMI 18 the same on every truck?
The general SCR-efficiency concern is similar, but thresholds, symptoms, test steps and repair criteria differ by manufacturer, engine family and emissions year.
Technical References
- Detroit Diesel service information: SPN 4364/FMI 18, GHG14
- International A26 supplemental diagnostics for SPN 4364 FMI 18
- How to read J1939 SPN and FMI fault codes
This guide is educational and does not replace the current OEM troubleshooting manual for the VIN, engine serial number and calibration installed on the vehicle.