Amen! Finally some is asking for the fundamentals of trouble shooting which would take much speculations away. Please post temperature and pressure data since impacted flow is the most common reason for error code 4. I also disagree with Curt, I have seen open systems without freeze protection and no flow seen frozen shut within 10-15 seconds (may be this is a northern problem....). Then the system can fool you since ice has built up on the inside of the coil, but there is still a lumen, but there is a high pressure delta, suggesting a high flow, when flow is actually less due to the smaller lumen, causing repeated shut downs. Simple way to test it is to run the unit in A/C mode (one of the reasons why they are recommended to be started in A/C when commissioned). If you hear the ice banging down in the pipes, you got yourself a hint.