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 DD13 aftertreatment system uses selective catalytic reduction to convert NOx in the exhaust into nitrogen and water by injecting diesel exhaust fluid into the exhaust upstream of the SCR catalyst. The ACM continuously monitors the efficiency of this conversion by comparing NOx concentrations at the SCR inlet and outlet. When SPN 4364 FMI 18 is set, the ACM has determined that the SCR conversion efficiency is below the moderately severe threshold — the aftertreatment system is not processing NOx at the required rate. On the DD13, this fault most commonly results from DEF quality problems, a DEF dosing injector that is not delivering the correct volume, an SCR catalyst that has been degraded by oil contamination or thermal events, or a NOx sensor that is providing inaccurate readings. The ACM may initiate an inducement sequence with progressive derates if the fault is not addressed.
SPN 4364 is Aftertreatment 1 SCR Conversion Efficiency; FMI 18 indicates valid data below normal at the moderately severe level. On GHG17 DD13 engines, the ACM calculates SCR efficiency from upstream (SCR inlet) and downstream (SCR outlet) NOx sensor data. Detroit DiagnosticLink provides live data for both NOx sensor positions, the DEF dosing actuator, the DEF quality sensor, and SCR temperature data. The DiagnosticLink DEF dosing actuation test confirms whether the injector opens and closes correctly. DEF concentration verification with a refractometer provides ground-truth DEF quality data independent of the on-board sensor. The DD13 GHG17 inducement sequence and timer data are visible in DiagnosticLink to show the current derate level and remaining time before escalation.
Common Symptoms
- Check engine and aftertreatment warning lamps on
- SCR efficiency or DEF system warning on the driver display
- Engine derate or speed limitation if inducement has advanced
- Unusual DEF consumption relative to expected rate for the duty cycle
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF quality below specification — urea concentration outside the 32.5% +/- 1.5% range
- DEF dosing injector clogged or providing insufficient DEF volume
- SCR catalyst degraded by oil contamination, thermal damage, or age
- Upstream or downstream NOx sensor failure or calibration drift
- DEF pump or supply circuit fault reducing DEF availability to the injector
First Checks
- Test a DEF sample with a refractometer to confirm urea concentration is within specification
- Connect Detroit DiagnosticLink and view live NOx sensor data at SCR inlet and outlet positions
- Perform the DiagnosticLink DEF dosing injector actuation test to confirm the injector is operating correctly
- Check the DEF supply circuit — filter, pump pressure, and supply lines — for restriction or crystallization
- Review inducement timer status in DiagnosticLink to determine how far the derate sequence has progressed
Can I Keep Driving?
The inducement timer may be running. Address DEF and dosing system faults promptly to prevent derate escalation.
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
Does SPN 4364 FMI 18 on the DD13 mean the SCR catalyst needs replacement?
Not automatically. SCR catalyst replacement should be considered only after DEF quality has been confirmed in-spec, the DEF dosing injector has been tested and is delivering the correct volume, and the NOx sensors have been verified to be reading accurately. A catalyst that is showing low efficiency due to a DEF delivery problem can appear to be failed but will recover normal function once the DEF quality and dosing system are corrected. Catalyst replacement is warranted if efficiency remains below threshold after all other corrections have been made.
Can engine oil in the exhaust cause SPN 4364 FMI 18 on the DD13?
Yes. Engine oil entering the exhaust from worn piston rings, valve stem seals, or a turbocharger oil seal leak can deposit phosphorus compounds on the SCR catalyst surface, reducing the catalyst's ability to convert NOx. This is called catalyst poisoning. If SPN 4364 FMI 18 is accompanied by blue smoke from the exhaust and higher-than-normal oil consumption, investigate and correct the oil consumption issue before catalyst replacement, as a new catalyst will be poisoned again if the oil consumption continues.
Will the Detroit DD13 fail an emissions inspection with SPN 4364 FMI 18 active?
An active SPN 4364 FMI 18 indicates the check engine lamp (MIL) is illuminated and an emissions-relevant OBD fault is stored. In jurisdictions that use OBD-based emissions testing, a vehicle with an illuminated MIL is an automatic test failure. Correcting the root cause, clearing the fault code, and completing the required OBD drive cycle to confirm emissions monitors are ready is necessary before the vehicle can pass an OBD-based emissions inspection.