Technician was out on October 13 to install hot water recirculating pump under manufacturer warranty. Technician appears to have polled wire out of butt connector for hot water thermistor on the outlet side. Appears to be fault of technician, no charge to customer for service and repair today. Installed new thermistor and turned water and power supply back on , no leaks or error codes. Also tankless water heater does not have thermal expansion tank per manufacturer requirements.