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Error code reference

Kenmore Dryer Error Code F-40

Communication between the machine control and motor control is lost.

Kenmore codesKenmore dryer codes

Communication between the machine control and motor control is lost.

First step from the service manual

See TEST #3b, page 8.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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What F-40 means on a Kenmore dryer

The F-40 error code on a Kenmore dryer indicates a loss of communication between the machine control board and the motor control board. These two control modules work together continuously during operation: the machine control board acts as the primary brain, sending commands to the motor control board, which in turn manages the drive motor speed and function. When the communication signal between them is interrupted or fails to respond within expected parameters, the dryer detects the fault and displays F-40 to alert the user and halt operation.

This type of communication failure can stem from several sources. The most common culprits are damaged or loose wiring harness connectors between the two control boards, a failed motor control board, or a failed machine control board. Corrosion on connector pins, a pinched wire, or a broken wire in the harness can all interrupt the data signal. In some cases, a voltage supply problem affecting one of the boards can also produce this code, since a board that is not receiving adequate power cannot maintain communication.

The recommended starting point for diagnosis is TEST #3b, which specifically targets the communication and control circuit between these two boards. This test will help isolate whether the fault lies in the wiring harness, the motor control board, or the machine control board itself. Technicians should inspect all connectors in the communication circuit for corrosion, looseness, or damaged pins before condemning either board.

Source: manufacturer service documentation.

Common causes of F-40

  1. 01

    Loose or disconnected wiring harness connector

    Wiring / connection

    The connectors linking the machine control board and motor control board can work loose over time due to vibration. Inspect both ends of the communication harness for connectors that are not fully seated or have pulled apart.

  2. 02

    Damaged or broken wiring in the harness

    Part replacement

    A wire in the harness between the two control boards may be pinched, chafed, or broken, interrupting the communication signal. Carefully trace the harness and look for visible damage or areas where wiring contacts the cabinet.

  3. 03

    Corroded connector pins

    Wiring / connection

    Moisture or age can cause corrosion on the connector pins at either control board, increasing resistance in the circuit and disrupting the data signal. Check pins for discoloration, oxidation, or residue that may indicate corrosion.

  4. 04

    Failed motor control board

    Part replacement

    The motor control board itself may have failed internally, causing it to stop responding to signals from the machine control board. If wiring checks out, the motor control board is the next component to evaluate per the TEST #3b diagnostic procedure.

  5. 05

    Failed machine control board

    Part replacement

    If the motor control board and all wiring test properly, the machine control board may be the source of the communication failure. This is typically the last component replaced after other causes have been ruled out.

See the test procedure for each cause

Frequently asked questions about F-40

What does F-40 mean on a Kenmore dryer?
The F-40 code means the dryer has lost communication between the machine control board and the motor control board. These two boards must exchange signals continuously for the dryer to operate. When that signal is interrupted, the dryer stops running and displays F-40. The fault can be caused by wiring problems or a failed control board.
Can I fix the F-40 error on my Kenmore dryer myself?
Some causes of F-40 can be addressed without professional help, such as reseating a loose wiring harness connector. However, diagnosing deeper wiring faults or replacing a control board requires working inside the dryer cabinet with the power disconnected and some comfort with electrical components. The diagnostic process starts with TEST #3b, which helps identify whether the problem is in the wiring or one of the control boards.
Will resetting the dryer clear the F-40 error code?
Unplugging the dryer for a few minutes may temporarily clear the code, but if the underlying communication fault is still present, the F-40 error will return. A reset does not fix a failed board or damaged wiring. It is only a useful step to confirm whether the fault is intermittent or persistent.
Is it safe to use my Kenmore dryer when the F-40 code is displayed?
No, you should not attempt to operate the dryer while F-40 is active. The dryer has shut down because communication between its control boards has failed, meaning it cannot properly manage the motor. Running the dryer in this condition could lead to further damage to the control boards or the drive motor.
How much does it cost to repair a Kenmore dryer with an F-40 error?
Repair costs depend on what component has failed. If only a wiring harness repair is needed, costs are generally lower, typically in the range of $50 to $150 for parts and labor. Replacing a motor control board or machine control board can bring the total to anywhere from $150 to $350 or more, depending on the specific board required and local labor rates.

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