SPN 1761 FMI 4 – DEF Tank Level Sensor Circuit Failed Low

This code indicates a low voltage signal from the DEF tank level sensor circuit. It suggests an electrical fault rather than a mechanical issue with the tank itself.

What This Code Actually Means

A 'failed low' condition signifies that the voltage signal from the DEF tank level sensor is below the expected range, typically indicating a short to ground, a broken ground reference, or an internal failure within the sensor itself.

What It Does NOT Mean

This does not necessarily indicate that the actual DEF level in the tank is low. The sensor may be malfunctioning and providing incorrect readings.

Common Real-World Causes on Freightliner Cascadia

Common issues include damaged wiring harnesses, corroded connectors, or a failed DEF level sensor. Water intrusion in the connector can also lead to this fault.

How the Truck Typically Acts

When this fault occurs, the driver may see a warning light on the dashboard, and the vehicle may enter derate mode if the system cannot determine the DEF level.

Step-by-Step Diagnostic Approach

Start by checking for proper power and ground at the sensor. Use a multimeter to verify the voltage levels. Inspect the wiring for shorts or breaks, and check the sensor itself for internal failure before replacing any parts.

Can You Keep Driving?

It's not advisable to continue driving with this fault, as it may lead to derate conditions and affect engine performance.

Shop Tip

Always check the connector for corrosion or moisture; this is a frequent culprit in electrical faults.