Diagnosing and Repairing False Thermostat Low Battery Error

Re-soldering a Broken Thermistor Solder Joint

It was a hot summer day when my thermostat quietly went from fan and A/C mode to fan-only mode. Instead of showing the current temperature, the display instead showed "LO", which turned out to mean low-battery. I figured that it may be as simple as replacing the batteries, but I was suspicious since the batteries were relatively new.

Battery and Power Diagnostics

I pulled both of the batteries out of the thermostat and measured their voltage with my digital multi meter. The batteries were 1.5V AA alkaline, and each was roughly 1.3V when tested. I considered that somehow the thermostat was picky and had a narrow voltage margin, though most devices will easily work when the batteries are far below 1.5V. I took out my variable power supply and hooked it up to the thermostat's battery leads. The batteries are in serial, so with two batteries it would expect around 3V. I started at about 2V out of curiosity and worked my way up to almost 4V. At all voltages, including ideal, the thermostat still displayed the low battery error, so the batteries weren't the problem.

Dismantling and Investigating

Since I figured out that the batteries themselves weren't the problem, that meant that the thermostat itself had an issue. I dismantled the thermostat and tested the continuity and resistance of the wires from the battery back to the board and found no issues. I was rather stumped at this point so I just looked and poked at everything on the board to try to find something out of place. I poked the blue thermistor, labelled RT1 on the board, and found that its left solder point had broken somehow.

Repair

I flipped the board so I had the solder points in view. I pulled out my soldering iron and applied some flux onto the solder joint on the board. I melted the solder using the soldering iron while pushing back the thermistor into the hole. The solder melted and the thermistor's connection was restored. I plugged the batteries into the thermostat to test and I no longer had the low battery error! By this point, the temperature had cooled down to the point of not needing A/C anyway, but I'd saved myself the time and money to get a replacement thermostat for this tricky error.