2002 Kia Sportage Check engine light on

My check engine light came on a month ago had it check at it read OP128. I went to my mechanic and he changed the thermostat and cleared the code. I came back on 2days later. Same code number. Went back to mechanic and the put temp. Sensor in and cleared the code. Came back on 4 days later same code. Left the code on. 9 days later check engine light goes off by its self. Goes back on 2 day later. Thermostat gage runs fine Cars runs fine. Any ideas?