95 metro code 15

I just bought a 95 geo metro with a 1 liter engine. the check engine light is on, the book i have says its a code 15 which is high coolant temp. but the gauge only goes over to about half way. i was wondering could this be a faulty sensor? the book i have doesn't go into much detail on how to diagnose anything it just tells you what the codes mean. the car has not been hot on me that i know of no boiling or anything like that.

any thoughts would be appreciated

I assume you already checked your coolant level. Could be a faulty sensor as you mentioned, or could be an old code in the ecm that needs to be cleared. Reset the ecm memory, typically by removing the tail light fuse for a minute, and see if the code 15 reappears.