how can i tell when its bad
gear position sensor on 1991 Ford Taurus
The only way to check this is with a voltmeter or diagnostic scan tool. You will needs the proper reference voltages to compare them to and that info is not always easy to find. According to the materials I have access to the light blue wire with yellow stripe should have 0 volts in park and 5 volts in drive. I will tell you, those sensors can be out of adjustment, so you might need to visit a shop that has the proper tools to check that and usually if the sensor is bad it will turn the check engine light on. Good luck.