Ariston Oven Error Code ER21

ER21 on an Ariston oven indicates that the NTC (Negative Temperature Coefficient) thermistor located on the power supply board is producing readings that the control system considers illogical. NTC thermistors are resistive temperature sensors whose resistance decreases predictably as temperature rises. When the control board monitors this sensor and receives values that fall outside the expected operating range or that contradict other system data, it flags the condition as ER21. This is distinct from a simple open or short circuit fault; the readings are present but inconsistent or nonsensical relative to the oven's actual operating state.

The NTC thermistor on the power supply board is used to monitor the thermal condition of the board itself, helping protect the electronics from overheating. If this sensor drifts out of calibration, develops a partial failure, or if the board's internal circuitry that processes the sensor signal becomes faulty, the resulting data fed to the main control logic will appear illogical. Wiring or connector issues between the power supply board and the main control board can also produce erratic resistance readings that trigger this error.

Because the NTC is physically integrated with or directly mounted on the power supply board, the recommended first diagnostic step is replacement of the power supply board as a complete assembly. Isolating the sensor from the board for independent testing is generally not practical in the field. Technicians should also inspect the wiring harness and connectors linking the power supply board to the main board before committing to a replacement, as a loose or corroded connection can mimic a failed board.