Water heater does not start heating, display shows F0 shortly after power-up

Safety first: Switch off the water heater using the main power switch and isolate it from the electrical supply. Close the gas valve if it is a gas-fired Protherm heater. Only qualified personnel should open the casing.

• Power reset: Turn the unit off for at least 2–3 minutes, then power it back on. Check if F0 clears.
• Check supply voltage: Verify that the supply voltage matches the rating on the Protherm rating plate. Over/under-voltage can corrupt parameters.
• Inspect control board connections: Remove the front cover, locate the main control PCB, and check all low-voltage harnesses and plug-in connectors for secure fit and signs of burning or moisture.
• Factory reset / parameter reload: If you have access to Protherm service documentation, enter the installer menu and reload default parameters for the specific model and gas type. Do not change gas type or maximum power without combustion analysis tools.
• Replace PCB if needed: If F0 persists after reset and wiring checks, the EEPROM or microcontroller on the main PCB is likely damaged. Replace the complete main control board with the exact Protherm spare part number.

Warning: Do not attempt to modify gas settings or hidden parameters without proper training and instruments. Incorrect settings can lead to unsafe combustion.