Peterbilt 389 / Cummins ISX, Paccar MX-13 etc. FAQsCummins ISX check engine light

Here are some common causes of inaccurate or erratic DEF level gauge readings on your Peterbilt semi truck with a Cummins ISX engine: Actually Low DEF fluid level: This may seem obvious, but if you’re near empty on your tank it can do weird things. This is the most common cause of inaccurate or erratic DEF level gauge readings. If the DEF fluid level is low, the gauge or dipstick may not accurately reflect the actual DEF level. Faulty DEF level sensor aka DEF sending unit: If the DEF level sensor or wiring is not functioning properly, it may not accurately detect the DEF fluid level and the vehicle's computer system may not display the correct DEF level on the dashboard gauge or dipstick. This can cause the DEF level to fluctuate or appear to be erratic. The DEF level sensor is part of a multi-purpose component that also includes the DEF quality sensor, DEF tank temperature sensor, and DEF tank heater. Please note if you are seeing DEF  fault codes for any of these components you may have a malfunctioning DEF sending unit aka header, which is mounted on top of the DEF tank and looks like a cap with a clamp, but has tubes and wiring that goes down into the tank.