Dunkirk Error A18: Maximum temperature error - high temperature supply switch
Lockout Alarm: the High Temperature (Supply) Switch opened during normal operation, indicating supply water exceeded its safe maximum. The page-54 tree checks switch continuity at J13-5/6 (two purple wires). Causes: low water flow, trapped air, low water/pressure, or a faulty High Temperature Supply Switch. Manual RESET required. We diagnose and repair it across Greater Vancouver — same-day and after-hours.
Lockout Alarm: the High Temperature (Supply) Switch opened during normal operation, indicating supply water exceeded its safe maximum. The page-54 tree checks switch continuity at J13-5/6 (two purple wires). Causes: low water flow, trapped air, low water/pressure, or a faulty High Temperature Supply Switch. Manual RESET required.
Dunkirk fault codes can vary by model and series. Use this as a guide — the same code can have more than one cause. Our Technical Safety BC–licensed technicians read the code, confirm it against your unit's manual, and diagnose the true cause before any repair.
Likely causes of Dunkirk error A18
- Low water flow (failed/air-bound circulator, closed valves)
- Air trapped in the heat exchanger
- System low on water / low pressure
- Faulty High Temperature Supply Switch
- Heat-exchanger pump wired/oriented wrong reducing flow
What you can safely check
- Check system pressure is in the normal 12-15 psi cold range
- Confirm the circulator pump is running and zone valves are open
Beyond these checks, do not open the unit or touch gas/electrical parts — BC law requires a licensed gas fitter. Call us for a proper diagnosis.
Error A18 usually leaves you without heat or hot water and can indicate a safety-related fault. Don't keep resetting it — call for priority after-hours Dunkirk repair.
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Parts often behind Dunkirk error A18
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Dunkirk Error A18 Repair Across Metro Vancouver
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