Hydrofarm says the probe is +/- 3ºF error. I have tested more than 5 (7 think now) and found they are all over the place. Not one was correct (I own a very accurate traceable thermometer) the worst was just over 3º high I would simply not trust the probe accuracy. The cheapest best thermometers on the market are found in the Proportional thermostats. Go figure. It is hard to break the +/-1ºF mark for less than 200$ but herpstat, helix both have done so. VE series has no published specs from the manufacturer that last time I checked so the assumption is +/-2º as it is the most common error range on the market.