F0 displayed, furnace does not start heating cycle

Safety first: Turn off power to the furnace at the main switch and close the gas valve before opening the unit. Wait several minutes for capacitors to discharge.

• Step 1 – Power reset: Turn off the furnace power for 2–3 minutes, then restore power. Check if F0 clears. If it returns immediately, continue.
• Step 2 – Inspect wiring harness: Remove the front cover. Locate the main control board (PCB) in the electronics compartment. Check all low‑voltage connectors (thermostat, sensors, communication bus) for loose or partially seated plugs. Reseat each connector firmly.
• Step 3 – Check configuration jumpers/dips: On many Protherm furnaces, the PCB has configuration jumpers or DIP switches (for gas type, fan mode, etc.). Compare their positions with the installation manual. Correct any obvious mis‑settings.
• Step 4 – Visual PCB inspection: Look for burn marks, swollen capacitors, or cracked solder joints on the main board. If any damage is visible, the board should be replaced, not repaired on site.
• Step 5 – Restore factory settings (if menu available): If the user interface allows, enter installer mode and perform a factory reset of parameters following the Protherm service manual. Do not change gas type or combustion settings unless you are certified.
• When to call a technician: If F0 persists after a reset and wiring check, the EEPROM or logic section of the PCB is likely defective. Replacement and re‑commissioning must be done by a qualified gas technician to ensure safe operation and correct parameter loading.