Replaced old customer front control panel to the Master Navien tankless water heater with new one. After replacement, ran test to make sure the master tankless was producing hot water by shutting off the other unit and only leaving the master unit. Ran only hot water in the kitchen sink for 5 minutes and fortunately hot water was indeed coming out. Also there were no error codes showing while the master unit. After the test it was determined the customer was good to use the unit again.