Atlantic heat-pump water heater displays E12; compressor does not start or stops quickly.

Safety: Isolate power. High-voltage and refrigerant components are hazardous; advanced diagnostics should be done by qualified personnel.

• Verify supply voltage at the heater terminals matches the rating plate (e.g., 230 V single-phase).
• Inspect the compressor contactor or inverter module for burn marks or loose connections.
• On non-inverter models, check the compressor start relay and capacitor; replace if visibly damaged or out of specification.
• Measure compressor winding resistances and insulation resistance to earth with appropriate instruments. If windings are shorted or open, the compressor must be replaced by a refrigeration technician.
• If the compressor is mechanically locked or the inverter module is faulty, contact an authorized Atlantic service center for compressor or inverter replacement.