There are many sensors, need to know what year, make, model and which sensor is in question.
It resembles a spark plug with wires attached.
The speed sensor is located on the transmission near the axle on the passenger side and lot of times the sensor gets corroded or the wires disconnect. If the wires and sensors look like they are in decent shape then most likely it is just a faulty sensor
bad crank shaft position sensor
On the exhaust near manifold to engine. look for a thing with a couple wires on a pipe.
look under the throttle body on the front of the engine there s a sensor with 3 wires going to it that is the engine coolant temperature sensor
the o2 sensor is in the exaust manifold or down pipe it looks like a sparkplug with wires
There is only one crank sensor fitted, you may also have a cam sensor. the crank sensor is located either around the bellhousing or around the front crank pulley. It either has 2 or 3 wires. It picks the signal off the revolving crank so look for a sensor in the block near flywheel.
this sensor sould be located on the inrake, look for your air box where the air filter is on the box or around it sould be a sensor on it you would notice by the wires cliping on to the sensor
Most likely the sensor is faulty. If you pull off the wire harness to the sensor and touch the 2 terminals, does the fan start up? If so, then just change the sensor. If not, then check the wires for current, and where you jumped the wires, look after that point for the problem.
It will have a plug to connect to the wires coming from the harness. This plug can be connected to the sensor with wires or sealed right into the sensor It will have an end that sticks into a hole in the engine. It may be directly held in the block or held in by clip but either way it will be held in by a screw or bolt.
on the front, top of the engine. look near the upper coolant hose on the engine for a sensor with a couple of wires on it. There may be 2 in close proximity that look similar.
If you have to crank and crank to get it started look for the coolant temperature sensor and change it. This is just one reason for hard start and is the most popular reason. Other reasons could be plugs, plug wires, coil pack, crank sensor, cam sensor and more.