Carrier 50NQ Installation, Start-Up And Service Instructions Manual page 14

UNIT SINGLE-PHASE COMPRESSORS are equipped
with a Compressor Start Thermistor (PTC device). When
supply voltage is within 10% limit and compressor does not
start, check the start thermistor with an ohmmeter.
Checking Start Thermistor
1. Shut off all power to unit and wait 10 minutes for
thermistor to cool to ambient temperature.
2. Remove thermistor from circuit.
3. Measure resistance of thermistor with ohmmeter. Nor¬
mal resistance readings are 12.5 or 25 ohms + 20% at
75 F ambient temperature depending on your unit.
4. If ohmmeter resistance reading is not within ± 20%,
the thermistor is defective and must be replaced.
If start thermistor is good and compressor does not start,
disconnect the thermistor from starting circuit and give
compressor a temporary capacitance boost. Run compressor
for 10 minutes, then shut off and allow system pressure to
equalize. Reconnect start thermistor and try restarting
compressor without boost capacitor. If after two attempts
the compressor does not start, remove thermistor and add
an accessory start capacitor and relay.