Recently my low tire pressure warning light came on. I filled all tires to proper pressure, 36 psi. Two tires were low at about 31.5 psi. Have now driven it for at least 50 miles but the light is still on. When I check the tire pressure on the screen, it shows me the pressure in three tires (36, 36, 37) but only has dashed lines for the fourth tire, left rear. Does this mean the sensor is bad? If so, can I change it myself and where do I go to get a new one? How much do they cost? Thanks for any help.