DEF Level & Temperature Sensor
Location, connector pinout, test procedure, and related fault codes for the DEF (Diesel Exhaust Fluid) level and temperature sensor on Class 8 diesel trucks.
Last updated: 2026-09-20
Overview
The DEF level and temperature sensor monitors the DEF tank fluid level and temperature. The ECM uses this data to manage DEF dosing, prevent DEF freezing, and trigger low-level warnings and inducement. A failed DEF sensor can cause false inducement or missed low-level warnings.
Sensor Locations by Platform
Cummins X15
Mounted inside the DEF tank, accessible from the top of the tank. Tank is typically located on the passenger side frame rail.
X15 DEF sensor requires Insite for sensor reset after replacement.
PACCAR MX-13
Inside DEF tank on passenger side frame rail. Sensor module mounts through top of tank.
MX-13 DEF sensor module includes heater — verify heater operation in cold weather.
Detroit DD15
Inside DEF tank, passenger side frame rail. Sensor module accessible from top of tank.
DD15 DEF sensor module includes integrated heater and quality sensor on some configurations.
Volvo/Mack MP8
Inside DEF tank on passenger side. Sensor module mounts through top of tank.
MP8 DEF sensor replacement requires PTT for sensor reset.
DieselVision™ identifies sensor labels and connector pin numbers — upload a close-up photo and the system maps pins automatically.
Connector Pinout
| Pin | Function | Specification |
|---|---|---|
| 1 | Power Supply (12V) | 11.5–14.5V |
| 2 | Ground | <0.1V to chassis ground |
| 3 | CAN High or Signal | 2.5–3.5V (CAN) or 0.5–4.5V (analog) |
| 4 | CAN Low (if CAN type) | 1.5–2.5V (CAN) |
Test Procedure
Read DEF level percentage in live data
Expected: Should match actual DEF level in tank. Fill tank and verify reading increases
Tool: OEM diagnostic software
Read DEF temperature in live data
Expected: Should match ambient temperature when cold. Normal operating range: 10–50°C
Tool: OEM diagnostic software
Check power supply voltage at sensor connector
Expected: 11.5–14.5V
Tool: Multimeter
Check ground circuit resistance
Expected: <0.5 ohms to chassis ground
Tool: Multimeter
Inspect DEF tank for DEF crystallization around sensor
Expected: Sensor and tank should be free of white DEF crystal buildup. Crystals can jam sensor
Tool: Visual inspection
Verify DEF quality — contaminated DEF can cause sensor faults
Expected: DEF should be clear, not yellow or cloudy. Use DEF quality tester if available
Tool: DEF quality tester (refractometer)
Common Failure Modes
- •DEF crystallization jamming sensor float or ultrasonic element
- •Sensor internal failure — stuck level reading
- •CAN communication fault to sensor
- •Contaminated DEF causing false readings
- •DEF heater failure causing frozen DEF in cold weather
- •Connector corrosion from DEF exposure
Frequently Asked Questions
Where is the DEF level sensor located?
The DEF level and temperature sensor is mounted inside the DEF tank, accessible from the top of the tank. The DEF tank is typically on the passenger side frame rail.
What causes a DEF level sensor to read incorrectly?
Common causes include DEF crystallization jamming the sensor, contaminated DEF, sensor internal failure, or CAN communication faults.
Can I drive with a failed DEF level sensor?
A failed DEF level sensor can trigger inducement (derate to 5 mph) if the ECM cannot verify DEF level. Address the fault promptly to avoid inducement.
What fault codes are caused by a failed DEF level sensor?
Common fault codes include SPN 1761 FMI 1 (DEF level below normal), SPN 1761 FMI 18 (DEF level below threshold), and SPN 5245 FMI 31 (DEF inducement active).
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