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Error code reference

LG Range Error Code C-21

Oven heating over temperature (appears when oven reaches 650°F/343°C in any cooking function except self…

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Oven heating over temperature (appears when oven reaches 650°F/343°C in any cooking function except self clean).

First step from the service manual

Disconnect power, open back cover, disconnect sensor harness, measure sensor resistance (1080 ohms at room temperature); check resistance of broil, bake, and convection heater; check DLB, Broil, Bake, and Convection heater relays.

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

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What C-21 means on a LG range

The C-21 error code on an LG range indicates that the oven cavity has reached or exceeded 650°F (343°C) during a standard cooking function. This threshold is intentionally set below the self-clean operating temperature, so the code only triggers during bake, broil, or convection modes. When the oven temperature climbs beyond this limit, the control board interprets the condition as a dangerous overheat event and halts operation to protect the appliance and the user.

The oven temperature sensor (RTD probe) is the primary component in this diagnostic. At room temperature, the sensor should measure approximately 1080 ohms of resistance. A sensor that reads significantly lower resistance can cause the control board to misread cavity temperature, potentially allowing the heating elements to run unchecked. Beyond the sensor, the broil, bake, and convection heating elements themselves should be checked for proper resistance values, as a shorted element can produce excessive, uncontrolled heat output.

The relay board is also a critical suspect in C-21 diagnostics. The Double Line Break (DLB) relay, along with the individual broil, bake, and convection heater relays, controls power delivery to each heating element. A relay that has welded contacts will remain closed and supply continuous power to an element even when the control board commands it off. This stuck-on condition prevents the oven from cycling properly and can drive cavity temperatures well above normal operating limits. A thorough diagnosis requires checking all four relay circuits after disconnecting power and removing the back cover.

Source: manufacturer service documentation.

Common causes of C-21

  1. 01

    Stuck or welded heating element relay

    Part replacement

    A relay on the control or relay board can fail in the closed position, keeping an element energized continuously even when the board tries to shut it off. Check the DLB, broil, bake, and convection heater relays for welded contacts after disconnecting power.

  2. 02

    Faulty oven temperature sensor

    Part replacement

    If the RTD sensor drifts out of its specified resistance range (approximately 1080 ohms at room temperature), the control board may receive inaccurate temperature data and allow elements to overheat. Disconnect the sensor harness and measure resistance with a multimeter to verify.

  3. 03

    Shorted or malfunctioning heating element

    Part replacement

    A bake, broil, or convection element that has internally shorted can draw excessive current and generate heat beyond normal levels. Measure the resistance of each element and compare against the expected values to identify a faulty heater.

  4. 04

    Damaged sensor wiring or harness

    Wiring / connection

    Burned, pinched, or corroded wiring between the temperature sensor and the control board can cause erratic resistance readings that mimic an overtemperature condition. Inspect the harness connector and wiring run for visible damage after disconnecting power.

  5. 05

    Failed control board

    Part replacement

    If the relays, sensor, elements, and wiring all test within spec, the control board itself may be misreading sensor input or failing to open relay circuits correctly. This is typically considered after all other components have been ruled out.

See the test procedure for each cause

Frequently asked questions about C-21

What does C-21 mean on a LG range?
C-21 means the oven cavity temperature exceeded 650°F (343°C) during a standard cooking function such as bake, broil, or convection. This is an overtemperature protection error. The range shuts down heating to prevent damage or a safety hazard. It does not appear during self-clean mode, which is designed to operate at higher temperatures.
Is it safe to use my LG range when the C-21 code is showing?
No, you should not continue using the oven while C-21 is active. The code indicates the oven reached a dangerously high temperature, which points to a component failure that has not been resolved. Continued use before diagnosing and repairing the root cause could result in further appliance damage or a fire hazard.
Will resetting the power clear the C-21 error on my LG range?
Disconnecting power may temporarily clear the code, but it will not fix the underlying problem. If a relay is welded shut, a sensor is faulty, or an element is shorted, the condition that caused the overtemperature event will still be present. The code is likely to return once the oven is used again unless the root cause is repaired.
How do I fix the C-21 error on my LG range?
Start by disconnecting power, removing the back cover, and measuring the resistance of the oven temperature sensor at the harness connector. It should read approximately 1080 ohms at room temperature. Next, check the resistance of the bake, broil, and convection heating elements, and inspect the DLB and individual heater relays for signs of failure. Repair or replace whichever component tests outside of its acceptable range.
How much does it cost to repair a C-21 error on an LG range?
Repair costs vary depending on which component has failed. An oven temperature sensor typically runs in the range of $20 to $60 for the part. A relay board or control board can cost anywhere from $80 to $250 or more. If you hire a professional technician, expect to add labor costs on top of parts, which can bring the total to $150 to $400 depending on your area and the specific repair needed.

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