# How to Fix Whirlpool Error F77 (Inlet Thermistor Fail)
If your Whirlpool washing machine (often seen on Duet or similar combo models) suddenly stops mid-cycle and displays the F77 error code, the machine has detected a fault in its water temperature sensing system.
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## 1. Explanation: What is Whirlpool Error F77?
The F77 error code specifically indicates an **Inlet Thermistor Failure**.
A thermistor is a small sensor used to measure temperature. In your washing machine, the Inlet Thermistor monitors the temperature of the water entering the drum. This information is crucial for the control board (CCU) to regulate heating and ensure the machine is operating safely and accurately based on the cycle you selected (e.g., Cold Wash, Sanitary, etc.).
**In plain English:** The sensor that tells your washer how hot the incoming water is has failed, is disconnected, or is providing an inaccurate reading. The machine will lock up to prevent potential issues like overheating or inaccurate wash temperatures.
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## 2. Step-by-Step Fix: Diagnosing and Replacing the Sensor
Because this error relates directly to a sensor reading failure, the troubleshooting process involves checking connectivity and then confirming the component failure using a multimeter.
### Safety First!
**Always unplug the washing machine from the power outlet and turn off the water supply before attempting any diagnostic steps or repairs.**
### Step 1: Perform a Hard Reset
Before opening the appliance, try clearing the error by performing a hard reset. This can sometimes resolve temporary glitches caused by minor power fluctuations.
1. Unplug the machine completely.
2. Wait 5 to 10 minutes.
3. Plug the machine back in.
4. Attempt to run a diagnostic cycle or a quick wash to see if the error returns.
### Step 2: Access the Control Board and Sensor
You will need to access the internal components, which usually requires removing the top panel of the washing machine.
1. **Locate the Inlet Thermistor:** Depending on your model, the thermistor is typically located near the inlet water valve assembly where the hoses attach, or sometimes integrated near the heating element inside the drum.
2. **Inspect the Wiring Harness:** Visually check the wires leading from the sensor back to the main control board (CCU). Look for visible signs of damage, fraying, loose connections, or rodent chewing. Secure any loose plugs.
### Step 3: Test the Thermistor for Resistance (The Critical Test)
If the connections appear fine, the sensor itself is likely faulty and must be tested using a multimeter set to Ohms ($Omega$).
1. **Disconnect the Sensor:** Unplug the connector leading directly to the thermistor.
2. **Place Meter Probes:** Place the probes of the multimeter onto the two terminals of the thermistor itself.
3. **Check Resistance:** You are looking for a specific resistance value (typically thousands of Ohms, or kΩ). While the exact resistance varies by model and temperature, a healthy thermistor should provide *some* reading.
* **Signs of Failure:**
* **OL (Open Loop/Infinite Resistance):** This usually indicates the sensor is broken internally.
* **0 (Zero Resistance/Short Circuit):** This indicates the sensor is shorted out.
* **A highly illogical reading:** If the sensor reads 100,000 Ohms in a cold environment when it should read 10,000 Ohms, it has failed.
If the multimeter confirms the sensor is open (OL) or shorted (0), the part must be replaced.
### Step 4: Replacement
If the thermistor failed the resistance test, it must be removed and replaced with a new component.
1. **Remove the Old Sensor:** Carefully unclip or unscrew the existing thermistor from its mounting point. Note its orientation before removal.
2. **Install the New Sensor:** Install the replacement part, ensuring it is seated securely.
3. **Reconnect Wiring:** Reconnect the wiring harness firmly.
4. **Reassemble:** Replace the panels and plug the machine back in to test.
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## 3. Recommended Replacement Part
While the F77 error references the “Inlet Thermistor,” the underlying component that often fails or is listed for replacement in many Whirlpool service manuals is categorized by parts suppliers as a general thermistor, sometimes crossover listed as the Dryer Thermistor, especially in combo units.
**Check the price for a replacement Dryer Thermistor here: https://www.amazon.com/s?k=Whirlpool+Dryer+Thermistor&tag=trivian-20**
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