Checking an air conditioner temperature sensor (thermistor)
An air conditioner has several temperature sensors (thermistors): room, coil, compressor discharge, outdoor air. A sensor's resistance changes with temperature and the board reads it. When the value is out of the plausible range (open, short, or far off), the unit reports a sensor error.
How to check
Find the sensor the code points to (each sensor has its own code) and its connector on the board.
Power off and unplug the sensor from the board.
Measure the resistance across the connector pins, and the actual temperature where the sensor sits.
Compare with the resistance-temperature table in the manual of the exact model - each brand and sensor type has its own table.
Infinite: the sensor or its wire is open. Near zero: shorted. Far off the table: the sensor is faulty.
If the sensor passes and the error stays: check the connector (corrosion, loose), the wire, then the board's input.
A pipe sensor must sit firmly in its place with its clip and insulation as fitted - a misplaced sensor reads wrong even when it is good.
Safety: switch off both the indoor and the outdoor unit before opening a cover or touching a board; the capacitors on the outdoor board stay charged for a while after power-off - wait for the board's lamps to go out and measure before touching. Refrigerant and high-voltage work belongs to a certified technician.
How to read an air conditioner error code Where the code shows, why one code means different things for different brands and models, and what to do before looking it up.