Question: I have a TH Pen 8708 Thermo-Hygrometer, which I've found to be really accurate. However it's now showing an "E5" error message and this is not covered in the user manual at all. Can you direct me on how to fix this, or send me a service manual?

I remove the battery and then inside the housing of the meter, you need to short out both the positive and negative contacts together (not on the battery but inside the meter)
I would gently clean the sensor with electronics cleaner and flow with gentle air compressor air.
This will clear the error and then install a brand new battery.
Per your request we have updated the webpage will all the latest information including the owner's manual.
No additional information beyond what is posted is available at this time.