What brand/type of chiller? Mine reads about 2 degF higher than the tank. Most chillers have the ability to calibrate the Temp. Also, if it is a flow through, I would check for calcium buildup and flush it with vinegar. Might need to get a replacement Temp probe or control circuit?

Oh yea, how much (GPH) water are you pumping through it (if a flow through), probably has a min GPH rating?