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Error code reference

LG Microwave Error Code F-1

Toaster heater or thermistor fault

LG codesLG microwave codes

Toaster heater or thermistor fault — indicates defective heater, defective thermistor, or loose connection at CN3 connector.

First step from the service manual

Check continuity of thermistor, then check continuity of heater, then check the connection between CN3 connector and PCB assembly.

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

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What F-1 means on a LG microwave

The F-1 error code on an LG microwave indicates a fault within the toaster heater circuit or its associated thermistor. This code triggers when the control board detects an abnormal signal from the thermistor, which is a temperature-sensing resistor responsible for monitoring the heat output of the toaster heater element. When the thermistor reads a value outside the expected resistance range, or when the heater itself fails to draw proper current, the PCB assembly registers the fault and displays F-1 to prevent unsafe operation.

There are three primary causes the control system flags under this code: a defective heater element, a defective thermistor, and a loose or failed connection at the CN3 connector on the PCB assembly. The thermistor's resistance value changes predictably with temperature, so an open circuit, a short, or a drifted resistance reading will all trigger the fault. Similarly, a heater element that has burned out or developed an open circuit will fail a continuity check and confirm the component as the source of the fault.

Diagnosis should proceed in a specific order. First, check the thermistor for continuity and verify its resistance falls within the expected range at room temperature. Next, check the heater element for continuity, as an open reading confirms a failed element. Finally, inspect the CN3 connector where the thermistor and heater circuit interface with the PCB assembly, looking for corrosion, pushed-back terminals, or a loose seating. Reconnecting or reseating CN3 sometimes resolves the fault without replacing any components.

Source: manufacturer service documentation.

Common causes of F-1

  1. 01

    Loose or failed CN3 connector

    Wiring / connection

    The CN3 connector links the heater and thermistor circuit to the PCB assembly. A loose, corroded, or partially disconnected CN3 can interrupt the signal and trigger F-1 without any component actually being defective.

  2. 02

    Defective thermistor

    Part replacement

    The thermistor monitors heater temperature by changing its electrical resistance. A failed thermistor will show an open circuit or an out-of-range resistance reading during a continuity check, confirming it as the fault source.

  3. 03

    Burned-out heater element

    Part replacement

    The toaster heater element can burn out over time, resulting in an open circuit. A continuity test directly across the heater terminals will show no continuity if the element has failed.

  4. 04

    Damaged wiring between thermistor and PCB

    Wiring / connection

    Wiring in the heater circuit can crack, fray, or develop poor connections over time, particularly near high-heat areas. Visual inspection of the wiring harness connected to CN3 can reveal breaks or heat damage.

See the test procedure for each cause

Frequently asked questions about F-1

What does F-1 mean on a LG microwave?
The F-1 code on an LG microwave indicates a fault in the toaster heater circuit or the thermistor that monitors it. The control board has detected either a defective heater element, a defective thermistor, or a loose connection at the CN3 connector on the PCB assembly. The appliance displays this code to alert you that the heating system requires inspection before normal operation can continue.
Can I fix the F-1 error on my LG microwave myself?
A knowledgeable DIYer can start by inspecting and reseating the CN3 connector, which requires no replacement parts and sometimes resolves the fault entirely. Testing the thermistor and heater element for continuity requires a multimeter and the ability to safely access internal components. Because microwave interiors contain high-voltage capacitors that can hold a dangerous charge even when unplugged, internal repairs should only be performed by someone familiar with high-voltage safety procedures.
Is it safe to use my LG microwave when F-1 is displayed?
No, you should not continue using the microwave while F-1 is active. The code indicates the heater circuit is not functioning correctly, and the control board has flagged the fault to prevent unsafe operation. Continuing to run the appliance in this state could cause further damage to the PCB assembly or other components.
How much does it cost to repair the F-1 error on a LG microwave?
Repair costs depend on the failed component. If only the CN3 connector needs reseating, there may be no parts cost at all, though a service call fee typically ranges from $50 to $100. Replacing a thermistor generally runs $20 to $60 for the part, while a heater element replacement can range from $30 to $80 or more, plus labor. Total repair costs including professional service typically fall between $80 and $200 depending on the component and your location.
Will resetting my LG microwave clear the F-1 error code?
Unplugging the microwave and plugging it back in may temporarily clear the F-1 display, but the code will return if the underlying fault is still present. A reset does not repair a defective thermistor, a burned-out heater element, or a loose CN3 connection. The fault must be diagnosed and the root cause corrected to permanently resolve the error.

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