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Reverse Cycle Heat AC
I’m not certain I understand your question - “you replaced the thermostat sensor, & the unit worked fine”.
I’m going to presume that, having replaced the thermostat, your unit still does not heat ???
Reverse Cycle Heat/Cool units have a Reversing Valve, that must be periodically energized, in order to prevent it from seizing up. To keep the Reversing Valve operating freely, switch the A/C to Heat mode, for a few moments, at least once a month.
You might be able to “Free-Up” a seized Valve, by gently tapping it, while in Heating Mode.
The less likely problem with the Valve could be a broken coil. You can test this (with ALL POWER OFF) by removing the two (2) Reversing Coil Wires from the terminal strip (in the control box - not the thermostat), and measuring for continuity with your Ohmmeter . The coil should read near Zero Ohms.
Good luck, & regards,