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Power off the machine and LOTO. Check the spindle motor terminal box for loose, corroded, or damaged thermistor connections (typically labeled TH1/TH2 or similar).

Verify zero voltage before touching any connections.

At the spindle amplifier, inspect the temperature sensor connector (consult amplifier manual for specific terminals). Look for bent pins, corrosion, or loose connections.

Measure thermistor resistance at the motor terminal block with an ohmmeter. A healthy PT100 or PTC thermistor should read approximately 100-150 ohms at room temperature. Open circuit (infinite) or short (zero) indicates failure.

If resistance is normal at motor but alarm persists, check continuity through the entire cable run from motor to amplifier to isolate cable damage.

Disconnect thermistor leads at the motor terminal box and measure resistance directly at the motor thermistor terminals. Compare reading to motor nameplate specifications (typically PT100, PT1000, or PTC type).

Ensure motor has cooled to ambient temperature for accurate baseline reading.

If thermistor reads open circuit (infinite resistance) or significantly out of range, the motor thermistor has failed and motor must be replaced or sent for rewind with new thermistor installation.

As a temporary diagnostic test only, check spindle amplifier parameters for thermistor type setting (parameter 4020-4025 range depending on system). Verify it matches the actual motor thermistor type.

Review alarm history for previous overtemperature alarms (SP-055, SP-074) which may indicate thermal stress that damaged the thermistor.

After verifying thermistor and wiring are intact, check amplifier diagnostic LEDs and use ladder or maintenance screens to view raw temperature input values. No reading or fixed value suggests amplifier fault.

Substitute a known-good thermistor or precision resistor (matching motor thermistor value) directly at the amplifier terminals to simulate motor sensor. If alarm clears, wiring or motor thermistor is faulty; if alarm persists, amplifier is suspect.

Use only appropriate resistance values - incorrect simulation can damage amplifier input circuitry.

Inspect amplifier for visible damage, burnt components near temperature input terminals, or signs of moisture/contamination on PCB.

If amplifier fault is confirmed, replace the spindle amplifier or contact Fanuc for repair/replacement options. Amplifier board-level repair of temperature circuits is typically not field-serviceable.