Detroit Diesel SPN 3364 FMI 1 – DEF Concentration Below Specification

Code Details

Plain-English Meaning

Detroit's GHG14 and GHG17 aftertreatment systems depend on DEF that is close to 32.5% urea by weight. The quality sensor — located in the DEF tank module — continuously monitors this concentration. When the reading drops below the acceptable band, the ECM records SPN 3364 FMI 1 and begins counting distance toward an inducement derate. On Kenworth T680s, Freightliners, and other trucks running the DD15, this code shows up most often after a DEF fill at an unfamiliar location or after the DEF cap fails to seal properly, allowing water intrusion.

SPN 3364 is the J1939 parameter for Aftertreatment 1 DEF Concentration. FMI 1 indicates data valid but below the normal operational range. Detroit uses an ultrasonic DEF quality sensor integrated into the DEF tank module. The sensor also measures fluid temperature, which affects the concentration reading. DiagnosticLink displays the current DEF concentration reading and the sensor temperature, allowing a technician to compare against the acceptable range at a given fluid temperature.

Common Symptoms

• DEF warning lamp or check engine lamp active
• Inducement distance counter shown in the driver display or through telematics
• No immediate power restriction at the early stage of inducement
• Possible DEF quality message on the dash depending on vehicle upfit

Possible Causes

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

• DEF tank filled with water or the wrong type of fluid
• Water intrusion through a damaged or missing DEF cap or filler neck seal
• DEF that has degraded — DEF older than one year or stored improperly degrades
• DEF quality sensor failure, particularly after high-temperature exposure or if the sensor has been submerged
• Wiring harness fault between the DEF tank module and the aftertreatment control module

First Checks

• Connect DiagnosticLink and read the current DEF concentration value — compare to the specification at the current fluid temperature
• Drain a sample of DEF from the tank and inspect it: fresh DEF is clear with a slight ammonia odor; discolored, cloudy, or odorless fluid suggests contamination
• Inspect the DEF fill cap for cracking or missing seal; check the fill neck area for water staining or ice residue
• Check whether the DEF level is very low — extremely low levels can cause the sensor to read outside its normal range even with good fluid
• If fluid quality checks out, test the sensor using DiagnosticLink's component tests to verify sensor output accuracy

Can I Keep Driving?

The truck is typically driveable when this code first sets, but the inducement counter is running. Depending on how long the code has been active and the current inducement stage, the remaining distance before derate may be limited. Check DiagnosticLink as soon as practical to see where the inducement counter stands.

Related Lookup Pages

• SPN 3364 topic page
• FMI 1 explanation
• Detroit Diesel manufacturer hub
• Aftertreatment fault codes
• Search this code