Getting Code P1281 w/check engine light. Could new thermostat be faulty ?

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2001 Dodge Ram Quad Cab Sport, Recently had thermostat housing replaced and I believe he replaced thermostat as he was supposed to. Check engine light on, and code P1281 comes up using trip odometer method. Engine not hot enough, The light came on before it should have reached normal operating temp going by where needle usually sits on the gauge. It was about 80 degrees outside, and I had only driven about a mile at 35 to 40 MPH when check engine light came on. I thought it may have been my gas cap because I had got gas the night before and had a hard time with my locking gas cap after adding fuel. Then I only drove about 1/2 mile home and no light was on. Then it came on next evening as I passed the place where I got gas the night before. I thought maybe that was a sign it was the gas cap, but decided to turn into next driveway and check code cuz destination was about 70 miles roundtrip, and theres a whole lotta nothin' 'tween here and there. Code P1281 so went home instead.