The fault could be lack of hydraulic fluid, low brake fluid in the reservoir, a broken hydraulic line or a bad master brake cylinder. The sensor is used to detect pressure differentials in the hydraulic system. If the car alerts a fault in the hydraulic system and the system checks out, the sensor itself may have failed.^[2]

The sensor has an input port to measure the hydraulic braking pressure and an electrical connector output for a wire connection for feeding the warning light. Some brake failures do not trip the sensor into a fault mode.

A brake fluid pressure sensor is used in anti-lock braking system (ABS). ABS fault lights come on with unexpected wheel lock from the ABS system, but could also include low fluid in the reservoir.^[3][4][5][6]

New accident avoidance technologies system also use brake fluid pressure sensors.^[7]

Many large heavy vehicles, like trucks, particularly those having multiple trailers use air brakes that use air pressure sensors.

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  Mechanical Pressure Switch with integrated connector
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  Mechanical Pressure Switch with Plug-in connector

• Hydropneumatic
• List of auto parts
• Pascal's law