SPN 1761 FMI 3 – DEF Tank Level Sensor Circuit Failed High
This fault indicates a high reading from the DEF tank level sensor circuit, typically pointing to an open circuit or a short to the 5V reference. Common causes include an unplugged sensor, a broken signal wire, or harness damage.
What This Code Actually Means
The "failed high" status usually suggests that the signal wire is either open or shorted to the 5V reference voltage. When the ECM detects a voltage higher than the expected range from the sensor, it triggers this fault.
What It Does NOT Mean
This does not indicate a problem with the DEF tank itself or the fluid level. It's strictly related to the electrical circuit of the sensor.
Common Real-World Causes on Freightliner Cascadia
Common issues include damaged wiring in the harness, corrosion at the connector, or a faulty sensor. In my experience, it's often the harness that gets pinched or frayed, especially in areas where it rubs against metal components.
How the Truck Typically Acts
When this fault is active, the DEF system may go into a reduced power mode, and you might see a warning on the dash. The truck may also experience issues with DEF dosing.
Step-by-Step Diagnostic Approach
- Check the power and ground at the sensor connector.
- Verify the 5V reference is present.
- Inspect the signal wire for continuity and shorts to ground or 5V.
- Examine the connector for corrosion or damage.
- If everything checks out, replace the sensor.
Can You Keep Driving?
It's not advisable to continue driving with this fault active, as it can lead to further issues with the DEF system and potential derate.
Shop Tip
Always inspect the wiring harness thoroughly before replacing the sensor; many times, the wiring is the culprit, and replacing the sensor won't fix the issue.
