If your heater is not heating correctly please check the following: STEP 1 Check for any burnt components/wires and be sure all the connections are secure. If any components are burnt, please go to NOTE 1. STEP 2 Check AC line voltage to be sure it is correct, (L1 to L2, L2 to L3 and L1 to L3). Then check L1, 2 and 3 to ground. STEP 3 Be sure your controller is configured correctly according to the O&M manual and the settings on the sticker inside your heater. STEP 4 If applicable, please check the LED on the low water cut off board in your heater. The green LED should be on indicating there is water in the heat chambers. STEP 5 Be sure you have water flowing and go into diagnostic mode by pressing the up and down arrows together for 1 second and let go. The display will start scrolling through different numbers where F=flow rate gpm, i=inlet temperature, o=outlet temperature. If the F is 000 then the flow meter is not recognizing flow and it will not heat. Please remove the flow meter and clean or replace.