In order to fix a Navien tankless water heater error code E438, you can try the following: Check the circulation pump: Make sure the pump is not damaged and is functioning properly. Check the flow sensor: Clean the sensor if there is any scale or debris buildup. Check the water supply and return lines: Make sure they are open and clear of any obstructions. Check the 2-way valve: Make sure it is positioned correctly to allow proper water flow. Descale the unit: You may be able to descale the unit