DEF Quality Sensor

WARNING: This page is about a different car, the 2018 Nissan Titan XD. However, it is still accessible from the selected car via links, so may be relevant.

A DEF quality sensor is mounted on the side of the DEF tank. This sensor detects the urea concentration in the DEF to confirm it is acceptable for effective dosing (approximately 32.5% urea). If the DEF quality sensor detects an incorrect DEF concentration, the DEF warning lamp in the combination meter will illuminate. In some cases, the MIL may also illuminate.

Courtesy of NISSAN NORTH AMERICA, INC.
If the DEF tank is filled with anything other than DEF or has diluted DEF, the after treatment system will not operate correctly. A DTC will be set and engine power reductions will follow.