If the probe reads HH:

This could be an indication of a low or weak battery. Ensure that you are using recommended batteries:

• Verify you are using fresh batteries in the sensor. Lithium batteries are recommended for installation locations where temperatures can go below -4°F/-20°C.
• Verify you are using batteries with a nominal voltage of 1.5 volts.          
  NOTE: Some rechargeable batteries carry low or inconsistent voltages that may not properly power the device.  
• Mixing different battery types (brands, old/new, etc.) is not recommended.

If this doesn't resolve the issue, a replacement probe is recommended.