SPN 1761 FMI 31 – DEF Tank Level 3 - Low
This code indicates that the DEF tank is at a low level, triggering a warning in the system. It’s important to differentiate between electrical and mechanical issues when diagnosing.
What This Code Actually Means
This fault is triggered when the DEF tank level sensor reads below a certain threshold. The sensor operates on a voltage signal that varies with the level of DEF in the tank. If the voltage drops below the expected range, the ECM flags this code.
What It Does NOT Mean
This code does not necessarily indicate a faulty sensor. It can also be a result of low DEF fluid or issues with the tank itself.
Common Real-World Causes on Freightliner Cascadia
- Low DEF fluid level due to consumption or leaks.
- Faulty DEF level sensor or wiring issues.
- Corrosion or damage in the DEF tank.
How the Truck Typically Acts
The truck may enter a derate mode if the DEF level is critically low, affecting performance and fuel efficiency.
Step-by-Step Diagnostic Approach
- Check DEF fluid level visually.
- Inspect wiring and connectors for damage or corrosion.
- Test the voltage output from the DEF level sensor.
- Verify the ground connections and power supply to the sensor.
- If all checks are normal, consider replacing the DEF level sensor.
Can You Keep Driving?
It’s not advisable to keep driving if the DEF level is low, as it can lead to derate conditions.
Shop Tip
Always check for software updates on the ECM, as they can sometimes resolve sensor-related issues without hardware changes.
