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Error code reference

LG Microwave Error Code F-9

Fault related to inverter module, magnetron, relay, or unsafe AC input voltage

LG codesLG microwave codes

Fault related to inverter module, magnetron, relay, or unsafe AC input voltage — oven does not cook despite display showing set figures and START pad being touched.

First step from the service manual

Check the connection between magnetron, door & control PCB and inverter module.

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

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  • Step-by-step tests with expected resistance, voltage, and continuity values
  • Wiring diagram references and connector pinouts from the OEM service manual
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What F-9 means on a LG microwave

The F-9 error code on an LG microwave indicates a fault within the high-voltage cooking circuit, specifically involving one or more of the following components: the inverter module, the magnetron, a relay, or the incoming AC supply voltage. Unlike conventional transformer-based microwaves, LG inverter models use an inverter PCB to regulate power delivery to the magnetron continuously rather than cycling it on and off. When the control system detects that the magnetron is not receiving or converting power correctly, or that a relay is failing to close the cooking circuit, it triggers the F-9 fault and halts the cooking cycle even though the display continues to show the programmed time and power settings.

The inverter module is a primary suspect because it directly controls the high-voltage output fed to the magnetron. A failed or failing inverter board can prevent magnetron excitation entirely while leaving the control interface fully functional. The magnetron itself can fail internally, presenting an open or shorted filament, which the inverter module detects as an abnormal load condition. Relay failures, particularly the main power relay on the control or inverter PCB, can interrupt the circuit path before power even reaches the inverter stage. Unsafe or unstable AC input voltage, such as voltage that is too low or too high at the wall outlet, can also trigger this fault as a protective shutdown.

The first diagnostic step is to inspect all wiring harness connections between the magnetron, the door and control PCB, and the inverter module. Loose, corroded, or burned connector pins in this circuit path are a common root cause and should be ruled out before replacing components. After confirming connection integrity, component-level testing of the inverter board output voltage, magnetron filament continuity, and relay switching function is required to isolate the fault.

Source: manufacturer service documentation.

Common causes of F-9

  1. 01

    Loose or faulty wiring connections

    Wiring / connection

    Disconnected, corroded, or burned connector pins between the magnetron, door and control PCB, and inverter module are a leading cause of F-9. Inspecting and reseating all harness connectors in this circuit is the first recommended step.

  2. 02

    Failed inverter module

    Part replacement

    The inverter PCB regulates high-voltage power delivery to the magnetron. If the board fails internally, the magnetron receives no power and the control system registers an F-9 fault. A failed inverter board typically requires full board replacement.

  3. 03

    Defective magnetron

    Part replacement

    The magnetron can fail with an open filament, internal short, or degraded cathode, causing the inverter module to detect an abnormal load and shut down. Filament continuity can be checked with a multimeter, but a shorted magnetron requires replacement.

  4. 04

    Relay failure on control or inverter PCB

    Part replacement

    A relay responsible for closing the cooking circuit can fail open, preventing power from reaching the inverter stage entirely. This is identifiable by listening for the relay click at the start of a cook cycle and by testing relay continuity.

  5. 05

    Unstable or unsafe AC input voltage

    Wiring / connection

    Input voltage at the wall outlet that falls outside the acceptable range can trigger the F-9 fault as a protective shutdown. Verify that the outlet is supplying correct voltage and that the microwave is not sharing a circuit with high-draw appliances.

See the test procedure for each cause

Frequently asked questions about F-9

What does F-9 mean on a LG microwave?
The F-9 code on an LG microwave indicates a fault in the high-voltage cooking circuit involving the inverter module, magnetron, a relay, or the incoming AC supply voltage. The microwave displays set cook times and responds to the START pad, but no actual cooking occurs because power is not being delivered to the magnetron correctly. The fault is the control system's way of signaling that the component responsible for generating microwave energy has failed or lost its power path.
Can I fix the F-9 error on my LG microwave myself?
Some causes of F-9, such as loose wiring harness connections or an unstable wall outlet, can be addressed without replacing parts. However, components like the inverter module, magnetron, and relays involve or are adjacent to lethal high-voltage capacitors that can hold a dangerous charge even after the unit is unplugged. Unless you are trained in high-voltage appliance repair and know how to safely discharge capacitors, this repair should be handled by a qualified technician.
Is it safe to continue using my microwave when the F-9 code is showing?
The microwave is not producing heat when F-9 is active, so there is no immediate cooking hazard. However, the underlying fault involves the high-voltage inverter and magnetron circuit, and continued use attempts should be avoided. Using a malfunctioning high-voltage appliance risks further component damage and should not be continued until the fault is diagnosed and repaired.
Will resetting my LG microwave clear the F-9 error code?
Unplugging the microwave for a minute and restoring power may temporarily clear the displayed code, but if the underlying hardware fault persists, the F-9 error will return as soon as you attempt to run a cook cycle. A reset is only useful as a diagnostic step to determine whether the fault is intermittent. A permanent fix requires identifying and repairing the defective component in the inverter, magnetron, relay, or power supply circuit.
How much does it cost to repair an LG microwave with an F-9 error?
Repair costs depend heavily on which component has failed. Wiring connection repairs or outlet corrections are low cost, often under $50 in labor if a technician is involved. An inverter module replacement typically runs between $120 and $250 in parts and labor combined. Magnetron replacement can range from $100 to $300 or more depending on parts pricing and labor rates. In some cases, especially on older units, the total repair cost may approach the cost of a replacement microwave, so getting a diagnostic estimate first is advisable.

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