Denso Air Conditioner Error Code E21

E21 indicates a power device damage abnormality where the compressor startup current or voltage was detected at or below the specified threshold. The system monitors these values during startup and trips this fault when they fall outside the acceptable range. It is a protective shutdown designed to prevent further damage to the compressor and power electronics. A relay board replacement followed by fan motor initialization is the prescribed first diagnostic step.

No, continuing to operate the unit with an active E21 fault is not recommended. The system has detected abnormal power conditions at the compressor, and continued operation could cause further damage to the power module, compressor, or connected components. The unit typically shuts down automatically when this fault is active as a protective measure. Leave the system off until the fault has been properly diagnosed and repaired.

Repair costs will vary depending on which components need to be replaced and local labor rates. If the relay board is the primary failure, parts and labor combined typically range from moderate to high cost, roughly in the range of $150 to $500 or more depending on part pricing and technician fees. If the compressor itself is damaged, costs increase significantly. Getting a diagnostic assessment from a qualified technician before authorizing repairs is a good way to understand the full scope of the work needed.

This fault involves high-voltage power components including the relay board, compressor circuit, and inverter electronics, which present serious safety risks if handled without proper training. The repair requires replacing the relay board and performing a fan motor initialization procedure, both of which require technical knowledge and appropriate tools. It is strongly recommended to have a qualified appliance or HVAC technician perform this repair. Attempting it without experience could result in injury or additional component damage.

A simple power reset is unlikely to permanently clear E21 because the underlying fault involves a hardware issue with the relay board or power device. The error may temporarily disappear after a reset but will return once the system attempts to start the compressor and detects the same abnormal current or voltage conditions. Clearing this fault requires replacing the faulty relay board and completing the fan motor initialization to address the root cause.