Samsung Heat Pump Error Code E121

E121 indicates that the outdoor unit PCB has detected either a reversed phase sequence or a missing phase in the 3-phase power supply. The PCB monitors all three phases to protect the compressor and fan motors from running on incorrect or incomplete power. The system shuts down automatically when this condition is detected. A technician should measure the voltage at connector CN82 across all three terminal pairs to begin diagnosing the fault.

No, the heat pump will not operate while E121 is active because the PCB shuts the system down upon detecting this condition. This shutdown is a protective function that prevents compressor and motor damage from reverse phase rotation or single-phase operation. Do not attempt to bypass or reset the protection circuit and resume operation until the underlying power supply issue has been corrected.

Repair costs vary depending on the cause. If the fault is a reversed phase connection, the cost is primarily a service call fee, typically in the range of $75 to $200, since the fix involves rewiring rather than replacing parts. A blown fuse replacement is inexpensive at $10 to $50 for parts, plus labor. If the outdoor unit PCB has failed and requires replacement, costs can range from $300 to $800 or more depending on parts and labor rates in your area.

Diagnosing and repairing a 3-phase power fault involves working with 380V AC electrical systems, which is dangerous without proper training and equipment. A licensed electrician or certified HVAC technician should perform voltage measurements at the outdoor unit PCB, check phase wiring order, and inspect fuses and breakers. Homeowners should not attempt to open electrical panels or disconnect boxes to reorder phase conductors.

Resetting the unit will not permanently clear E121 if the root cause has not been corrected. The phase detection circuit will re-evaluate the incoming power on startup and will trigger E121 again if the phase order is still reversed or a phase is still missing. The fault must be resolved at the electrical supply level before the unit will operate normally.