Pentair Pool Heater Not Heating: E05, E01, and Flow Switch Diagnosis

Pentair pool heaters (MasterTemp, Max-E-Therm) display E-codes on the control panel. E05 = stack flue sensor limit (exhaust temperature too high from low water flow or gas pressure issue). E01 = water temperature sensor fault. E02 = water pressure switch open (same as Hayward LO - clean pool filter). Pentair's specific advantage: the MasterTemp has a hydraulic bypass built into the header, unlike Hayward where the bypass is external. If the internal bypass valve is stuck open, water shortcuts through the heater without being heated. This is diagnosed by feeling the outlet pipe - if it is the same temperature as the inlet after the heater runs for 5 minutes, the internal bypass is stuck. Pentair also uses a different ignition system than Hayward: the MasterTemp uses a hot surface igniter (like a furnace) rather than a standing pilot.