Bosch tankless water heater shows E1 and will not ignite; burner never lights

Safety first: Turn OFF electrical power and close the gas shutoff valve. Do not attempt repair if you smell strong gas; ventilate the area and contact your gas supplier or emergency services.

• Check gas supply: Ensure the manual gas shutoff valve to the heater is fully open. Confirm other gas appliances in the home work normally. If multiple appliances are affected, contact your gas supplier.
• Inspect ignition electrodes: Remove the front cover. Locate the ignition electrodes near the burner. Look for carbon buildup, cracks, or loose connections. Gently clean the electrodes with fine emery cloth and ensure the gap and position match the service manual.
• Clean flame sensor: The flame rod is often combined with the igniter or located adjacent to it. Remove light surface oxidation with fine sandpaper and wipe with a clean dry cloth. Reinstall firmly.
• Check wiring: Inspect the harness from the control board to the gas valve and ignition assembly for loose plugs, corrosion, or damaged insulation. Reseat connectors.
• Test restart: Restore gas and power, then open a hot water tap to demand heat. If E1 returns, the gas valve or control board may be defective and should be tested and replaced by a qualified Bosch technician.