F26 displayed, furnace does not modulate correctly, may lock out

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Defective modulating gas valve on Protherm burner, Broken modulation coil wiring, Faulty PWM/0–10V output from PCB, Incorrect gas valve calibration

Safety first: Turn off power and gas. Only a licensed gas technician should adjust or replace gas valves.

Step 1 – Visual check: Inspect the modulation connector on the gas valve for loose pins or damaged wires. Reseat the plug.
Step 2 – Technician tests: A technician should measure the modulation signal from the PCB and the response of the valve. If the valve does not respond correctly, it must be replaced.
Step 3 – Re‑commissioning: After valve replacement, the technician must set gas pressures and verify combustion with an analyzer.

Professional Required 5/5

Modulating Gas Valve

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