Code Details
| Display code | SPN 4364 FMI 18 |
|---|---|
| SPN | 4364 |
| FMI | 18 |
| OEM code | None listed |
| Manufacturer | Mack |
| 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 Mack MP8 SCR aftertreatment system uses diesel exhaust fluid injected upstream of the SCR catalyst to convert NOx emissions into nitrogen and water. The ECM measures the SCR conversion efficiency by comparing NOx readings at the catalyst inlet and outlet. SPN 4364 FMI 18 is set when the measured efficiency falls below the moderately severe low threshold. On the MP8 — used in Anthem, Pinnacle, and Granite vocational trucks — DEF quality is a frequent cause of this fault because these vehicles operate in demanding environments where DEF tank contamination and quality variation are real concerns. Dosing injector faults and SCR catalyst degradation from oil contamination are additional causes. The ECM initiates a progressive inducement sequence that advances to derates if the fault is not corrected.
SPN 4364 is Aftertreatment 1 SCR Conversion Efficiency; FMI 18 indicates valid data below normal at the moderately severe level. On the Mack MP8 (GHG17 emissions platform shared with Volvo D13), the ECM calculates SCR efficiency from upstream and downstream NOx sensor data. Mack Premium Tech Tool (PTT) provides live data for both NOx sensors, DEF dosing actuation data, SCR temperature, and the inducement timer status. The PTT DEF dosing injector actuation test confirms whether the injector opens and closes correctly. A refractometer DEF quality test provides ground-truth data independent of the on-board DEF quality sensor.
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 the inducement sequence has escalated
- DEF consumption different from expected
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF quality below specification — diluted, contaminated, or degraded
- DEF dosing injector clogged or delivering insufficient DEF volume
- SCR catalyst degraded from oil contamination or age
- Upstream or downstream NOx sensor failure or drift
- DEF pump or supply circuit fault
First Checks
- Test DEF concentration with a refractometer — 32.5% +/- 1.5% specification
- Connect Mack PTT — check live NOx sensor data at SCR inlet and outlet
- Run the PTT DEF dosing injector actuation test
- Inspect DEF supply lines and filter for restriction or crystallization
- Check inducement timer status in PTT
Can I Keep Driving?
The inducement timer may be progressing. Address the root cause of low SCR efficiency promptly to prevent escalation to performance-limiting derates.
Related Codes
Related Lookup Pages
Sources
- Mack Trucks Service Support Public Resources Mack Trucks Inc. · oem · accessed 2026-06-11 · confidence medium
Source: Mack Trucks Inc., Mack Trucks Service Support Public Resources. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - Volvo Trucks / Mack Trucks Technical Support Volvo Trucks North America / Mack Trucks Inc. · oem · accessed 2026-06-10 · confidence medium
Source: Volvo Trucks North America / Mack Trucks Inc., Volvo Trucks / Mack Trucks Technical Support. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
Is SPN 4364 FMI 18 on the Mack MP8 the same fault as SPN 4364 FMI 0?
SPN 4364 FMI 18 and FMI 0 both indicate SCR conversion efficiency below normal, but at different severity levels. FMI 18 is the moderately severe level, while FMI 0 is the most severe level indicating the efficiency deficit has progressed further. FMI 18 typically appears earlier in an efficiency degradation trend, giving the operator a window to address the root cause before the more severe FMI 0 sets. The diagnostic approach is the same for both; the FMI level indicates the urgency.
Can a single leaking injector cause oil contamination of the SCR catalyst on the Mack MP8?
An engine injector with an internal fuel-to-return leak can cause excess fuel in the exhaust, but the primary concern for SCR catalyst contamination is oil. Oil reaching the exhaust from worn piston rings, valve stem seals, or a turbocharger seal can deposit phosphorus and other oil-derived compounds on the SCR catalyst washcoat, reducing its ability to convert NOx. If oil consumption is elevated on the MP8 alongside SPN 4364 FMI 18, investigate and correct the oil consumption source before SCR catalyst replacement.
Does the Mack MP8 share its SCR aftertreatment system with the Volvo D13?
Yes. The Mack MP8 and Volvo D13 are closely related engines that share a common aftertreatment architecture under the Volvo Group GHG17 platform. Diagnostic approaches for SPN 4364 FMI 18 are similar between the two engines, and both use Volvo Group aftertreatment components including the DEF dosing system. However, the Mack MP8 is diagnosed with Mack Premium Tech Tool (PTT) while the Volvo D13 uses VCADS Pro or Volvo Tech Tool.