Diagnostic Overview
Mack fault code P203C00 commonly indicates a low signal or circuit problem from the DEF/reductant level sensor. The ECM may think DEF level information is invalid, which can start inducement warnings and eventually limit power. Diagnosis should focus on the sensor circuit and tank header before assuming DEF quality or SCR failure.
Likely Causes
DEF level sensor low signal; tank header failure; wiring short to ground; connector corrosion; module or harness issue.
What To Check First
Inspect the DEF tank header, connector, and harness. Check live DEF level reading and circuit voltage.