LG Oven Error Code C-21

The C-21 error code on an LG oven indicates that the oven's internal temperature has risen to an abnormally high level, or that the oven temperature sensor has developed a short circuit while the oven is in operation. The temperature sensor, also called an RTD (resistance temperature detector) probe, is a thermistor-based component mounted inside the oven cavity. It continuously sends resistance readings to the control board, which uses those values to regulate heating element output and maintain the set temperature. When the sensor shorts internally, its resistance collapses far below the normal baseline, causing the control board to interpret the signal as an extreme temperature condition.

At room temperature, a properly functioning LG oven temperature sensor should measure approximately 1080 ohms. A shorted sensor will typically read significantly lower than this, often near zero ohms, while an open sensor reads infinite resistance. To begin diagnosis, disconnect power to the oven, locate the sensor harness connector at the back of the oven cavity or behind the rear panel, disconnect it, and measure resistance across the sensor terminals with a multimeter. A reading well outside the 1080-ohm baseline confirms sensor failure.

Beyond a faulty sensor itself, a damaged harness with wires contacting each other or the oven chassis can produce the same short condition. The control board can also contribute to this fault if it is unable to correctly process incoming sensor signals, though board failure is a less frequent cause. Thermal runaway, where the oven genuinely overheats due to a stuck relay on the control board, is another possible scenario that triggers C-21 without the sensor itself being defective.