Adam,
Your display unit appears to be perfectly fine - the readout is well within tolerance for 5% resistors. If you want to be sure, then use 1% resistors and take 3 measurements. You should be within 1 degree C using the following standard resistor values: 3000 ohms = 25C, 1000 ohms = 25C, 39000 ohms = -25C.

Glad to hear we're getting more of those outside temperature displays working again(at a reasonable cost)! Good luck,
Barrie