I replaced thermostat but now my Cooling fan constantly runs once I start truck an it gets to operating temp. What should I look at first?

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You can hear the fan running while driving.. It went from a pretty quite truck to being loud cause the fan is running wide open. But soon as I shut it off the fan stops. Once I start back up an hit operating temp its wide open. I had alot of air in the system after replacing thermostat but was sure I got all of it out cause the hot air is HOT now, an AC works fine too. Could it be maybe its low on coolant? Or am I looking at a sensor problem or something?