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Error Code
E2 points at a temperature sensor. The system has a reading that does not match what it expects, so it stops regulating properly and eventually faults out. In practice that is one of three things: the sensor itself has failed or drifted, the wiring to it is interrupting the signal, or something is sitting on the sensor and insulating it from the air it is meant to measure.
These are small thermistors that change resistance with temperature, and they drift as they age. A drifted sensor reads something plausible but wrong, which is more misleading than one that fails outright, because the system acts on a reading that looks reasonable. Replacement is inexpensive, and it is the usual answer once the wiring is cleared.
The sensor lead runs back to the board through a space that vibrates and collects condensate. A connector that has worked loose, corrosion on the pins or a conductor rubbed at a casing edge will change the resistance the board sees without the sensor being at fault at all. The board has no way to tell a cold pipe from a bad connection.
Dust matting on a sensor insulates it from the air it is measuring, so it reads the temperature of the dust rather than the room. The system then over or under runs, because it is chasing a figure that has nothing to do with the actual space. It sounds trivial, and it is a genuine cause on a unit that has never been cleaned.
The sensor is located, checked for dust coating and correct mounting, and cleaned if it is matted. Its resistance is then measured and compared against the temperature table for the room. A sensor that has drifted or reads open is replaced, and the new one is seated so it sits properly in the airflow.
The plug at the board is unplugged, inspected for corrosion and reseated, and the lead is traced for chafing along its run. Resistance is measured at the board end as well as at the sensor, so a difference between the two readings identifies the lead rather than the sensor.
Where cleaning and the wiring checks leave the reading still out of tolerance, the sensor is the fault and gets replaced. It is a low cost part, and fitting a new one is quicker than trying to prove an intermittent drift in the field. The code is then confirmed clear before the unit goes back into service.
