Cummins ISX15 / X15 SPN 4364 FMI 18 — SCR Conversion Efficiency Below Normal

Code Details

Plain-English Meaning

The ISX15 and X15 aftertreatment system uses selective catalytic reduction (SCR) to convert NOx emissions in the exhaust into harmless nitrogen and water. The SCR process requires diesel exhaust fluid (DEF) to be injected into the exhaust upstream of the SCR catalyst at a precisely metered rate. When the ECM compares the NOx concentration entering the SCR catalyst against the NOx concentration leaving the catalyst and finds that the conversion efficiency is below the expected level, it sets SPN 4364 FMI 18. FMI 18 indicates the moderately severe below-normal threshold. The most common causes are DEF quality issues (contaminated, diluted, or crystallized DEF), a DEF dosing injector that is not delivering the correct quantity of fluid, a degraded or poisoned SCR catalyst, or a faulty NOx sensor producing inaccurate efficiency readings. The ECM may initiate a derate or inducement sequence if the efficiency fault is not corrected.

SPN 4364 is Aftertreatment 1 SCR Conversion Efficiency; FMI 18 indicates valid data below normal range at the moderately severe level. On CM2350 ISX15 and X15 engines, the ECM calculates SCR conversion efficiency by comparing readings from the upstream and downstream NOx sensors. Cummins INSITE provides live NOx sensor data, DEF dosing rate data, SCR inlet and outlet temperature, and the DEF quality sensor reading. The INSITE DEF dosing injector actuation test confirms whether the injector is operating correctly. DEF quality can be verified using a DEF refractometer to check the urea concentration (32.5% +/- 1.5% is the specification). A degraded SCR catalyst typically shows a consistent efficiency deficit across all temperature and load conditions rather than an intermittent or temperature-dependent pattern.

Common Symptoms

• Check engine lamp and possible aftertreatment warning lamp illuminated
• SCR efficiency warning or inducement sequence notification on the driver display
• Engine derate if the inducement sequence has advanced
• DEF consumption change — either higher or lower than expected DEF usage
• Multiple aftertreatment fault codes present in INSITE

Possible Causes

Possible causes may include the items below. The list is not a parts diagnosis.

• DEF quality below specification — contaminated, diluted, or incorrect fluid in the tank
• DEF dosing injector clogged, failed, or not delivering the correct DEF quantity
• SCR catalyst degraded, poisoned by oil or fuel contaminants, or physically damaged
• Upstream or downstream NOx sensor failed or producing inaccurate readings
• DEF supply line, pump, or pressure circuit fault reducing DEF delivery

First Checks

• Drain a DEF sample and check urea concentration with a refractometer — specification is 32.5% +/- 1.5%; below or above this range indicates DEF quality is the cause
• Connect Cummins INSITE and view live NOx sensor data at both the upstream and downstream positions to determine whether the sensors are reading plausible values under current conditions
• Perform the INSITE DEF dosing injector actuation test to confirm the injector is opening and closing correctly and delivering DEF when commanded
• Check the DEF tank, supply line, and pump for contamination, crystallization, or flow restriction that could reduce DEF delivery volume
• Review the SCR inlet and outlet temperature history to ensure the SCR catalyst is reaching the operating temperature range required for efficient NOx conversion

Can I Keep Driving?

With SPN 4364 FMI 18 active, the engine may be operating under an inducement timer that will escalate to a derate if the efficiency fault is not corrected. Address DEF quality and dosing system faults promptly to prevent escalation.

Related Lookup Pages

• SPN 4364 topic page
• FMI 18 explanation
• Cummins manufacturer hub
• Aftertreatment fault codes
• Search this code