Code Details
| Display code | SPN 4364 FMI 18 |
|---|---|
| SPN | 4364 |
| FMI | 18 |
| OEM code | None listed |
| Manufacturer | Cummins |
| 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 Cummins ISB 6.7 uses a selective catalytic reduction system to treat NOx emissions by injecting DEF into the exhaust upstream of the SCR catalyst. The ECM monitors NOx concentrations using sensors positioned before and after the SCR catalyst to calculate conversion efficiency. When SPN 4364 FMI 18 is set, the ECM has determined that the ratio of NOx removed by the SCR catalyst is below the moderately severe efficiency threshold — meaning the aftertreatment system is not cleaning the exhaust to the required standard. On the ISB 6.7, which is widely used in medium-duty vehicles and school buses, DEF quality issues are a frequent cause because these vehicles are often refueled at multiple locations with varying DEF quality. A clogged DEF dosing injector or an SCR catalyst that has been contaminated by engine oil are additional common causes.
SPN 4364 is Aftertreatment 1 SCR Conversion Efficiency; FMI 18 indicates valid data below normal at the moderately severe level. On the Cummins ISB 6.7 (CM2350 B101), the ECM calculates SCR efficiency from upstream and downstream NOx sensor data. Cummins INSITE provides live data for both NOx sensors, the DEF dosing injector actuation rate, DEF tank level and quality sensor, and SCR inlet and outlet temperatures. The INSITE dosing injector actuation test verifies the injector opens and closes on command and that DEF is actually being injected. A refractometer DEF concentration test is the most direct method for confirming DEF quality independent of the on-board sensor.
Common Symptoms
- Check engine and aftertreatment warning lamps illuminated
- SCR efficiency warning or inducement notification on the instrument cluster
- Engine derate if inducement has escalated
- Unusual DEF consumption rate (higher or lower than typical for the duty cycle)
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF quality below specification — diluted, contaminated, or degraded urea solution
- DEF dosing injector clogged or not delivering the correct DEF volume
- SCR catalyst degraded from age, oil contamination, or thermal damage
- Upstream or downstream NOx sensor providing inaccurate readings
- DEF pump or pressure regulator fault reducing DEF delivery to the injector
First Checks
- Test the DEF with a refractometer to verify urea concentration is within 32.5% +/- 1.5% specification
- Connect Cummins INSITE and check live NOx sensor readings at both the upstream and downstream positions
- Perform the INSITE DEF dosing injector actuation test to confirm the injector operates correctly
- Inspect the DEF supply line and filter for crystallization or restriction that could reduce DEF delivery
- Review SCR catalyst temperature data in INSITE to confirm the catalyst is reaching the minimum operating temperature for efficient NOx conversion
Can I Keep Driving?
An active inducement timer may escalate to a derate. Address DEF quality and dosing system issues promptly to prevent inducement escalation and maintain emissions compliance.
Related Codes
Related Lookup Pages
Sources
- Cummins INSITE Service Tool — Public Reference Documentation Cummins Inc. · oem · accessed 2026-06-11 · confidence medium
Source: Cummins Inc., Cummins INSITE Service Tool — Public Reference Documentation. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - QuickServe Online Cummins Inc. · oem · accessed 2026-05-05 · confidence medium
Source: Cummins Inc., QuickServe Online. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
Can cold weather cause SPN 4364 FMI 18 on the ISB 6.7?
Very cold ambient temperatures can reduce SCR catalyst operating efficiency because the catalyst requires a minimum temperature to activate the urea-to-ammonia conversion reaction. If the ISB 6.7 is used primarily for short trips in cold weather where the aftertreatment system does not reach full operating temperature before the trip ends, SCR efficiency may temporarily fall below the fault threshold during the warmup period. INSITE SCR temperature data can show whether the catalyst is reaching adequate operating temperature during the driving cycles that set the fault.
How do I know if the DEF dosing injector is the cause of SPN 4364 FMI 18 on the ISB 6.7?
The INSITE DEF dosing injector actuation test commands the injector to open and confirms the DEF pressure response. A failed injector that does not open when commanded, or one that is partially clogged and delivering reduced DEF volume, will show an abnormal actuation test result. Additionally, if the DEF consumption rate on the vehicle is lower than expected for the mileage driven, reduced DEF delivery from the injector is a likely contributor. Physical inspection of the dosing injector for DEF crystallization at the tip is also useful.
Will flushing the DEF tank with fresh DEF resolve SPN 4364 FMI 18 on the ISB 6.7?
If the fault is caused by contaminated or out-of-specification DEF, draining the tank, flushing the system, and refilling with confirmed-quality DEF is an appropriate corrective action. After refilling, the SCR system needs time to operate with the new DEF before the ECM can confirm that efficiency has returned to acceptable levels. The inducement timer may need to expire and the SCR system efficiency may need to be recalculated through a drive cycle before INSITE shows the efficiency returning to the normal range and the fault becoming inactive.