It depends on the unit's condition and parts, and a good visit answers both in one trip. Wall heaters from the original builds can often be serviced, safeties, thermocouple, burner cleaning, but some models have outlived their parts supply, and a cracked or corroded firebox ends the discussion on safety grounds. If replacement is the answer, the modern options are a conventional furnace with ducts where the house allows, or ductless heat pump heads room by room. What you should not accept is a repair quote that does not mention the parts question, because that question decides whether the repair has a future.
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