Cummins SPN 4364 FMI 18 – SCR NOx Conversion Efficiency Below Normal

Code Details

Plain-English Meaning

The SCR (selective catalytic reduction) system reduces harmful NOx emissions by injecting DEF into the exhaust stream upstream of a catalyst. The ECM uses two NOx sensors — one before the SCR and one after — to calculate how much NOx the catalyst is removing. When the difference between those two readings is too small, it means the system isn't working efficiently enough. SPN 4364 FMI 18 is how the ECM reports that condition. It doesn't point directly at a single failed part; instead it reflects the net performance of the whole SCR subsystem — DEF fluid, dosing injector, catalyst, and sensors together.

SPN 4364 is Aftertreatment 1 SCR Conversion Efficiency in J1939. FMI 18 indicates the calculated efficiency ratio is valid but below the acceptable operating threshold at a moderate severity level. On Cummins CM2350 systems, the ECM calculates SCR efficiency from the upstream and downstream NOx sensor readings and compares the ratio against a calibrated minimum. When efficiency falls short and enough distance accumulates, the ECM advances the inducement sequence toward SPN 5246 FMI 31. This is a consequence code that reflects the combined performance of the entire SCR subsystem rather than a single component failure.

Common Symptoms

• Amber warning lamp or DEF/aftertreatment indicator active
• Inducement distance counter visible in Cummins Insite or the driver display
• No immediate driveability impact early in the inducement sequence, but escalating power restriction as distance accumulates
• May appear alongside SPN 3364 FMI 1 (DEF quality low) or dosing system codes

Possible Causes

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

• DEF quality below specification: diluted fluid, wrong fluid, or fluid degraded from age or heat cycling
• DEF dosing system fault: injector clogged, pump pressure low, or incorrect DEF quantity being delivered relative to exhaust conditions
• SCR catalyst degraded from thermal aging, sulfur poisoning, or hydrocarbon fouling from repeated failed or incomplete DPF regen events
• Upstream NOx sensor reading lower than actual NOx, making the catalyst appear less efficient than it is
• Downstream NOx sensor reading higher than actual NOx, artificially deflating the efficiency calculation
• Exhaust leak between the DEF dosing injector and the SCR catalyst, reducing the DEF concentration available for the catalytic reaction

First Checks

• Confirm DEF quality first: drain a small sample and check for clarity and odor — contaminated or wrong fluid is the most common field cause and the easiest to verify
• Connect Cummins Insite and read live NOx sensor values: compare upstream and downstream readings; if either sensor reads implausibly (near zero, both reading the same value, or wildly inconsistent), that sensor may be faulty
• Review dosing system status in Insite: check DEF dosing module output, DEF pump supply pressure, and injector test results
• Look for companion codes — DPF regen faults, exhaust temperature sensor codes, or dosing injector heater issues — as these can contribute to catalyst poisoning or misfiring that reduces conversion efficiency
• For high-mileage vehicles (400,000+ miles), consider whether the catalyst has reached end of life; a catalyst that passes DEF and dosing diagnostics but still shows low efficiency may need cleaning or replacement

Can I Keep Driving?

SPN 4364 FMI 18 starts an inducement countdown. Left unresolved, the truck will progress to a torque derate or speed cap (SPN 5246 FMI 31). The vehicle can continue operating in the early inducement stages, but planning for service soon prevents a situation where the truck is locked down at full inducement.

Related Lookup Pages

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