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Error code reference

LG Cooktop Error Code F9

Right inverter heat sink thermistor fault.

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Right inverter heat sink thermistor fault.

First step from the service manual

Check CN 02 (Right Module); thermistor should read 45~55 K ohms at room temperature.

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

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What F9 means on a LG cooktop

The F9 error code on an LG induction cooktop indicates a fault in the right inverter heat sink thermistor circuit. The heat sink thermistor is a temperature-sensing component mounted on or near the inverter module's heat sink. Its job is to monitor the thermal condition of the inverter board, which drives the induction coil. If the control system detects that the thermistor's resistance reading falls outside the expected range, it triggers F9 and shuts down that cooking zone to prevent heat damage to the inverter electronics.

Diagnosis begins at connector CN 02, which is the right module connection point. At room temperature, the thermistor should measure between 45 and 55 K ohms. A reading significantly above or below this range indicates a failed thermistor, a wiring problem between the thermistor and the board, or a damaged connector. Because this is an NTC (negative temperature coefficient) type thermistor, resistance drops as temperature rises, so an open circuit or a very high resistance reading points to a failed sensor rather than an overheating condition.

Common root causes include a thermistor that has drifted out of specification or failed outright, a loose or corroded connection at CN 02, damaged wiring in the thermistor harness, or in some cases an inverter board that has developed a fault affecting its thermistor input circuit. Physical inspection of the connector and wiring should always precede component replacement to rule out simple connection issues before committing to a more involved repair.

Source: manufacturer service documentation.

Common causes of F9

  1. 01

    Failed heat sink thermistor

    Part replacement

    The thermistor itself has drifted out of specification or failed open. Measuring resistance at CN 02 with a multimeter will show a value outside the 45 to 55 K ohm range at room temperature, confirming the sensor needs replacement.

  2. 02

    Loose or corroded connector at CN 02

    Wiring / connection

    The connector at CN 02 on the right inverter module can develop poor contact due to heat cycling or moisture, causing intermittent or false fault readings. Inspect and reseat the connector, checking for corrosion or bent pins.

  3. 03

    Damaged thermistor wiring harness

    Part replacement

    The wiring between the thermistor and the CN 02 connector can be pinched, chafed, or broken, resulting in an open or short circuit. A visual inspection of the harness routing and a continuity test will identify this issue.

  4. 04

    Faulty right inverter board

    Part replacement

    If the thermistor and wiring both test within specification, the inverter board's thermistor input circuit may have failed. This is less common but should be considered after ruling out the sensor and connections.

See the test procedure for each cause

Frequently asked questions about F9

What does F9 mean on a LG cooktop?
F9 indicates a fault in the right inverter heat sink thermistor circuit. The cooktop's control system monitors this thermistor to protect the inverter electronics from overheating. When the thermistor's resistance falls outside the expected range of 45 to 55 K ohms at room temperature, the unit displays F9 and disables the affected cooking zone.
Can I fix an F9 error on my LG cooktop myself?
A knowledgeable DIYer comfortable working with electronics can perform the initial diagnostics by checking the resistance at connector CN 02 with a multimeter. If the thermistor is the issue, replacing it is a moderately involved repair requiring disassembly of the cooktop. Work involving the inverter board should be handled carefully, as capacitors on the board can retain a charge even when unplugged.
Is it safe to keep using my LG cooktop with an F9 error?
The cooktop disables the affected right cooking zone when F9 is active, which is a protective measure to prevent damage to the inverter electronics. You should not attempt to bypass this shutdown. The cooktop should not be used until the fault is diagnosed and repaired, as continued attempts to operate it could cause further damage to the inverter board.
Will resetting my LG cooktop clear the F9 error?
Cycling power to the cooktop may temporarily clear the display, but F9 will return as soon as the control system re-reads the thermistor signal if the underlying fault has not been corrected. A reset does not fix the defective thermistor, wiring, or connector causing the code. The fault must be properly diagnosed and repaired to resolve the error permanently.
How much does it cost to repair an F9 error on an LG cooktop?
If only the thermistor requires replacement, parts and labor costs typically range from around 80 to 200 dollars depending on your location and the technician's rates. If the inverter board itself has failed, repair costs can range from 200 to 450 dollars or more, since inverter boards are more expensive components. Getting a diagnosis before committing to a repair is a good idea, as the cost of the board may factor into the decision to repair versus replace.

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