Code Details
| Display code | SPN 4364 FMI 18 |
|---|---|
| SPN | 4364 |
| FMI | 18 |
| OEM code | None listed |
| Manufacturer | Detroit Diesel |
| System | Aftertreatment — SCR |
| Component | SCR catalyst / DEF dosing system / NOx sensors / DEF quality |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-12 |
Plain-English Meaning
The Detroit DD15 SCR system injects diesel exhaust fluid into the exhaust stream upstream of the SCR catalyst to convert NOx emissions into nitrogen and water. The ACM monitors the conversion efficiency by comparing upstream and downstream NOx sensor readings. SPN 4364 FMI 18 indicates the efficiency has fallen below the moderately severe threshold. The DD15 is a high-output engine used in heavy linehaul applications where maintaining SCR efficiency is critical for both emissions compliance and avoiding inducement derates that could disrupt delivery schedules. DEF quality issues, dosing injector faults, and catalyst degradation are the most common causes. The ACM inducement sequence for SPN 4364 FMI 18 begins with warnings and escalates to derates if the fault persists.
SPN 4364 is Aftertreatment 1 SCR Conversion Efficiency; FMI 18 indicates valid data below normal at the moderately severe level. On GHG17 DD15 engines, the ACM calculates SCR efficiency from SCR inlet and outlet NOx sensor readings and compares the result against the efficiency threshold calibration. Detroit DiagnosticLink provides live NOx sensor data, DEF dosing system actuation data, SCR temperature data, and the inducement timer status. The DEF dosing injector actuation test in DiagnosticLink confirms whether the injector opens and closes on command. Refractometer testing of the DEF provides ground-truth quality data. The inducement timer in DiagnosticLink shows the current derate level and remaining time in the escalation sequence.
Common Symptoms
- Check engine and aftertreatment warning lamps on the instrument cluster
- SCR efficiency or DEF system warning on the driver display
- Engine derate if inducement has advanced beyond the initial warning stage
- Possible change in DEF consumption rate relative to normal expectations
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF quality below specification — contaminated, diluted, or old DEF
- DEF dosing injector clogged or not delivering the correct DEF rate
- SCR catalyst degraded from oil contamination, thermal events, or age
- Upstream or downstream NOx sensor inaccuracy or failure
- DEF supply circuit fault reducing DEF delivery to the injector
First Checks
- Test the DEF concentration with a refractometer — specification is 32.5% +/- 1.5% urea
- Connect Detroit DiagnosticLink and view live NOx sensor readings at both the SCR inlet and outlet positions
- Perform the DEF dosing injector actuation test in DiagnosticLink to confirm the injector functions correctly
- Check DEF supply line and filter condition for crystallization or restriction
- Review the inducement timer status to assess derate escalation urgency
Can I Keep Driving?
Inducement derate may be progressing. Correct DEF and dosing faults promptly. The DD15 linehaul inducement derate can severely restrict speed and torque if the fault runs through the full escalation sequence.
Related Codes
Related Lookup Pages
Sources
- Detroit Diesel Service and Diagnostic Resources — Public Reference Detroit Diesel (Daimler Truck North America) · oem · accessed 2026-06-11 · confidence medium
Source: Detroit Diesel (Daimler Truck North America), Detroit Diesel Service and Diagnostic Resources — Public Reference. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - DiagnosticLink Detroit Diesel Corporation · oem · accessed 2026-05-05 · confidence medium
Source: Detroit Diesel Corporation, DiagnosticLink. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
How does the DD15 inducement timer work with SPN 4364 FMI 18?
When SPN 4364 FMI 18 is set on the DD15, the ACM starts an inducement timer based on the EPA GHG17 aftertreatment inducement framework. The first stage is a warning with no derate; after a defined period of continued fault operation, the timer advances to the first derate level, and subsequent intervals advance to more severe derates. DiagnosticLink shows the current inducement stage, the timer countdown, and the derate level applied. Resolving the root cause and performing a proper reset procedure through DiagnosticLink stops the timer and ends the inducement sequence.
Can DEF that has been sitting in the tank over winter cause SPN 4364 FMI 18 on the DD15?
DEF that has been repeatedly frozen and thawed over winter is generally still usable if it was ISO 22241-compliant quality DEF to begin with, since DEF freezes and thaws without significant degradation. However, DEF that has been contaminated with water, dirt, or other fluids during storage, or that has been stored at extreme temperatures for extended periods, can degrade below specification. Testing the concentration with a refractometer is the definitive way to confirm whether stored DEF is still usable.
Is there a PM service interval for the SCR catalyst on the Detroit DD15?
The DD15 SCR catalyst does not have a fixed replacement interval in normal service — it is designed to last the life of the engine under proper operating conditions. However, the catalyst can be damaged prematurely by oil consumption introducing phosphorus compounds, fuel contamination, or severe thermal events. If SPN 4364 FMI 18 is traced to catalyst degradation after all other fault sources have been ruled out, inspection of the catalyst condition and potential replacement should be performed at a Detroit-authorized service location.