1995 Saturn 4door singlehead cam

I took my car to auto zone and had troubleshooting done. They told me it was code 15 and for me to check the coolant sensor/low temp,then check wiring harness connector at the PCM for damage. I have done all of this also replaced a relay,alternator and my engine light still comes on and sometimes the heat guage wil rise above halfway when it is hot outside. What else can I check for