Mechanical thermostat knob is stiff or cannot be turned.

Safety: Turn off power before attempting to adjust or remove the thermostat.

Steps:

• Remove knob: Gently pull off the plastic knob from the thermostat shaft.
• Inspect for melting: If the knob or surrounding plastic is deformed, investigate overheating causes (e.g., poor contact, high current).
• Test thermostat: With the knob removed, try turning the shaft gently with pliers. If it is seized, replace the mechanical thermostat.
• Install new knob: After replacing the thermostat, fit a new knob if the old one is damaged.