Sensors are provided in reservoirs on internal combustion engines, and other applications, in which the reservoir is subjected to variations in pressure and temperature that cause the reservoir to exert variable forces on the sensor, thereby affecting sensor accuracy. To overcome problems in the prior art, a reservoir and sensor cartridge system are disclosing having: a reservoir with first and second openings, a cap coupled to the first opening, and a sensor cartridge disposed in the second opening. The sensor cartridge includes a sensor disposed in a body, a circumferential indentation formed in an outer surface of the body, a seal disposed in the indentation, and a retention feature provided on the body's outer surface. The circumferential indentation with the seal is axially displaced along the body from the sensor.