2001 Dodge Caravan Sport. Check engine light is on....Sometimes the transmission will not shift into higher gears. Seems to be more prone to not shifting when hotter outside. Typically after setting overnight, shifting is fine/smooth but by afternoon, especially on hot days, shifting will not happen (On cool days/winter, shifting without problem). Fluids are fine. Van has check engine light on but code from auto zone read was useless (related to brakes per attendant)...does this sound like a sensor problem, e.g. transmission speed sensor and how difficult is this repair?