The E1/E2 error code is caused by the flame circuit not detecting flame at the burner.
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First, look for the information plate on the side of the water heater and confirm that your gas supply (Natural Gas or Liquid Propane) matches what is designated for the water heater. An incorrect gas pairing can result in incomplete ignition or it can cause the water heater to burn incorrectly.
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Second, confirm that the gas valve on your supply line is completely open and that there are no blockages or obstructions in the gas line.
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Contact a gas company to have your gas lines inspected and gas pressure measured. The gas pressure of Liquid Propane should be 8-11 inches of water column and the gas pressure of Natural Gas should be 4-6 inches of water column.