Unit compressor is running too slow

The FF thermister and Freezer thermister are the usual suspects. Cross-check their respective resistances according to the tech sheet. If they are both within tolerance, the PCB is likely bad. The thermistors that control the temps. on each side respectively. You said that previous company replaced them, but maybe they replaced only the defrost thermistors, they will be spliced in? I had same issue couple years back, the FF temp thermistor was the culprit. Try checking that voltage with the fan harness unplugged, also the sensor harness. Read the voltage at the proper pins at the board itself. If you find 5 VDC, the board is sending power to the thermisters. It could be a hall sensor on one of the fan motors causing the issue. I usually unplug the fan harness first and wait for a response from the inverter.