Geonics EM31 troubleshooting almost always starts with checking the batteries. Turn on the EM31 and set the knob to BATT to read the voltage. If the needle reads between the battery marks of an analog display or between +/- 4.4V on a digital display, then the batteries should not be an issue.
Note, the plus and minus values should be the same or very close to the same readings. If the readings are different, check the battery contacts. Geonics states “Through the normal course of operation, it is possible that the battery contacts will become contaminated with dirt and grit. To clean these contacts use fine sand paper (#400 or higher) and wipe several times over the contact. Ensure that the spring action of the battery holders is maintained. Bend holder sides slightly if necessary.” Sometimes, especially in shipping, the battery contacts get loosened due to the battery compartment being jostled in shipping. To correct this, take the batteries out, and pinch the connectors. Then reinsert the batteries as needed. Sometimes spinning the battery can help ensure proper contact as well.
Assuming the EM31 is not located over any metal or near any source of cultural interference, confirm the EM31 instrument is still reading negative values for apparent conductivity. Sometimes the only way to determine cultural interference is to pack up the instrumentation, drive 5 miles away, and test the equipment out at a new location.
If the EM31 still does not seem to be operating correctly, remove the transmitter and receiver booms.
High humidity and a large change in temperature may cause condensation inside the tube. Try using a small blow hair dryer to gentle warm and dry out the tube. The main body may also need to be dried out. Reassemble the transmitter and receiver to see if the EM31 yields positive apparent conductivity values.