replace that sensor that you unplugged. $30 ish (Cdn)with new seal (metal crush washer). Plug back in. Bob's your uncle! You should probably put in a new thermostat and o ring while your doing it. Also hoses would be wise.
I say this 'cause I did my rad ...then I had to do hoses...then I had to do sensor...then I had to to Thermsat. I could have saved a lot of time, money and cursing if I had odne it all at the same time.
Good luck
Out.