How do I troubleshoot a heat pump showing an E3 error code?

An E3 error signifies compressor low pressure protection or refrigerant undercharge/leakage. Check if the system's low pressure is lower than 7 psi, ensuring service valves are completely open. If necessary, charge refrigerant and check for pipeline leaks as per manufacturer instructions. Ensure normal operation by verifying correct wiring between components (Page 27).