Check Engine Light

my check engine light come n and off. ive narrowed it down to when the gas tank is half empty to 1/4 the light stays off?/ ive chnaged the gas cap twice. i saw a post that suggest the fuel tank pressure sensor? or maybe should i open it up clean out the pup and anything in the tank?? .also one post suggest fulel vapor line being checked and how would that be done?? this is a 1998 kia sportage

ALl of the above are relevant. It would have been interesting to have the code read .
THe pressure sensor connector is accessible from the trap under the rear seat. this sensor is analog
From the shop manual available at www.kiatechinfo.com : (test is made witn connector installed and engine running. )

4. Using a voltmeter, measure voltage at all three Fuel Tank Pressure Sensor terminals in the fuel tank sending unit connector.

---------------------------------------------------------------------------
WHT/BLK - 2.0 - 3.0 V (signal return voltage/base reading)
---------------------------------------------------------------------------
If signal voltage reads 5.0 volts, check the sensor ground wire.
---------------------------------------------------------------------------

GRN/YEL - 0.0 V (sensor ground)
YEL/GRN - 5.0 V (reference Voltage)