check you gauges, stamped into the back may be there temperature tolerance range. most are +- 1 degree. this is what you want. they may in fact be ok to use

if you feel they are still not accurate, it could be where the placement is. i like to keep mine close to the top right or left corners of the tank

if you do decide to get a to get another unit, i really like electronic 2 in one. i bought a chinese one off ebay for like $5, 6 months in and still working great