How do I test the defrost heater, defrost thermostat, and thermistor on a refrigerator using a multimeter?

Testing these three components with a multimeter allows you to pinpoint the exact failure point in an automatic defrost system without replacing parts unnecessarily.

Safety first: Unplug the refrigerator from the wall power outlet before removing any panel or handling electrical wiring.

1

Prepare the Refrigerator & Access Components

Safety and teardown

1. Unplug the refrigerator.
2. Access the interior back wall of the freezer compartment by removing the freezer shelves, drawer rails, and rear panel screws.
3. Carefully pull the panel forward to expose the evaporator coils, defrost heater, defrost thermostat (bimetal), and thermistor.

2

Test the Defrost Heater

Checking heating element continuity

The defrost heater is a metal tube or glass element running beneath or woven through the evaporator coils.

1. Unplug the wiring harness connectors from both ends of the heater. Always disconnect at least one terminal from the circuit to prevent back-feeding false readings.
2. Turn your multimeter dial to the Resistance / Ohms (Ω) setting (set to the 200Ω range if manual-ranging).
3. Touch one probe to each connector pin of the heater.

• Good Reading: Between 15 Ω and 120 Ω (depending on wattage). This confirms an intact continuous heating circuit.
• Bad Reading: O.L (Open Line/Infinite Resistance) or a reading above 1,000Ω. An O.L reading indicates a broken heater element that needs replacement.

3

Test the Defrost Thermostat (Bimetal Switch)

Temperature-dependent continuity test

The defrost thermostat is a clip-on metal disk snapped directly onto the top or side of the evaporator tube. It functions as a temperature-sensitive switch: open when warm, closed when cold.

1. Disconnect the thermostat’s wiring connector.
2. Warm Test (At Room Temp): Place multimeter probes on the two leads. It should read O.L (Open) because it prevents the heater from activating when the freezer is warm.
3. Cold Test (In-Circuit/Submerged): If the thermostat was just pulled off frost-covered coils (below 15°F / -9°C), or after soaking it in an ice bath for 5 minutes, test the leads again.

• Good Reading: Near 0 Ω (closed circuit/continuity) when cold; O.L when warm.
• Bad Reading: O.L even when frozen solid, or showing resistance values (e.g., 50Ω) instead of a clean open/closed state.

4

Test the Defrost / Temperature Thermistor

Checking temperature-variable resistance

The thermistor is a small plastic-encapsulated sensor. Unlike the bi-metal switch, its resistance changes continuously as the temperature changes (typically an NTC thermistor: higher resistance at lower temperatures).

1. Unplug the thermistor harness connector.
2. Turn multimeter to the 20k Ω range (or Auto-Ranging Ohms).
3. Measure the resistance across the thermistor leads at room temperature (~70°F / 21°C), then place it in a glass of ice water (32°F / 0°C) for a few minutes and test again.

• Good Reading:
  • At 70°F (21°C): ~2,000 Ω to 10,000 Ω (2k$\Omega$–10k$\Omega$, brand dependent).
  • At 32°F (0°C): Resistance should significantly increase (typically climbing to around 8,000 Ω to 30,000 Ω).
• Bad Reading: Displays O.L, 0 Ω (short circuit), or a static reading that does not change at all when cooled down.

Component Reference Table