# How to Fix Maytag Error F40 (Thermistor Open)
If your Maytag washing machine displays the F40 error code, it indicates a critical problem with the machine’s ability to monitor water temperature. This guide will help you understand, diagnose, and resolve this common issue.
—
## Explanation: What Error F40 Means
The F40 error code signifies a **Thermistor Open** fault.
**What is the Thermistor?**
The thermistor is a small, inexpensive temperature sensor typically located near the heating element or sump pump assembly. Its job is to constantly report the temperature of the water to the main control board (CCU). This ensures the water is heated to the correct temperature for the selected cycle.
**What does “Open” mean?**
An “open circuit” means that the flow of electricity to the sensor has been completely interrupted. The control board is looking for a resistance reading from the thermistor but is receiving none (an infinite reading).
**The F40 error is caused by one of three things:**
1. **Faulty Thermistor:** The sensor itself has internally failed. (Most common cause).
2. **Damaged Wiring:** A loose, corroded, or broken wire connecting the thermistor to the main control board.
3. **Control Board Failure:** Rarely, the control board (CCU) is unable to read the sensor input correctly.
—
## Step-by-Step Fix: How to Diagnose and Replace the Thermistor
Before attempting any repairs, ensure you have unplugged the machine from the power outlet and shut off the water supply.
### Step 1: Power Cycle and Visual Check
Sometimes, error codes are triggered by temporary glitches.
1. **Unplug the Washer:** Disconnect the machine from the wall for at least two minutes.
2. **Plug Back In:** Plug the machine back in and attempt to run a short cycle (like Drain/Spin). If the code immediately returns, proceed to the next steps.
### Step 2: Access the Thermistor Location
The thermistor is typically located on the tub or sump area, often adjacent to the heating element. You may need to tilt the machine back or remove the lower front panel, or access the component from the rear panel, depending on your specific Maytag model.
1. **Safety First:** Unplug the unit completely.
2. **Locate the Sensor:** Identify the small, often bullet-shaped sensor clipped or screwed into the bottom of the outer drum or sump housing. It will have two wires running into it via a small harness connector.
### Step 3: Inspect the Wiring Harness
Visually inspect the wires leading to the thermistor connector.
* Look for any signs of water damage, corrosion (green or white powdery buildup), or wires that have been chewed or melted.
* Gently disconnect the wiring harness from the thermistor and then reconnect it firmly. A loose connection is a very easy fix.
* If the wiring harness looks obviously damaged, you will need to splice in a replacement harness section or address the corrosion.
### Step 4: Test the Thermistor Resistance (Recommended)
To definitively prove that the thermistor has failed, you must test its resistance using a multimeter set to Ohms ($Omega$).
1. **Disconnect the Thermistor:** Completely unplug the wiring harness from the sensor terminals.
2. **Set Multimeter:** Set your multimeter to the **20k** (20,000) Ohms setting.
3. **Measure Resistance:** Place the multimeter probes onto the two terminals of the thermistor (it does not matter which probe goes on which terminal).
#### **Expected Results:**
| Condition | Expected Reading (Approximate) | Diagnosis |
| :— | :— | :— |
| **Room Temp (70°F)** | 10,000 to 12,000 Ohms (10-12 kΩ) | Sensor is likely good. The issue may be the wiring or the control board. |
| **Hot Water (140°F)** | Resistance should drop significantly (e.g., 2,000 Ohms) | Sensor is definitely good (it reacts to temperature change). |
| **F40 Failure** | **”OL” or “1” (Open Loop/Out of Limits)** | The thermistor has failed and must be replaced. |
If your multimeter reading shows “OL” or infinity, the circuit is open and the sensor needs replacement. Since the thermistor is inexpensive and the primary culprit for F40, replacement is the logical next step.
### Step 5: Replace the Thermistor
1. If the diagnosis confirms the thermistor failure, remove the old sensor. Depending on your model, this may involve gently pulling it out of a rubber grommet or unscrewing it.
2. Install the new thermistor, ensuring it is seated correctly and the gasket (if present) is properly sealed to prevent leaks.
3. Reconnect the wiring harness.
4. Reassemble the machine panels and restore power.
—
Check the price for a replacement Thermistor here: https://www.amazon.com/s?k=Maytag+Thermistor&tag=trivian-20