O5
Buderus Safety Circuit Open (Limit, Door Switch, or External Safety)

O5 code, furnace does not start, safety chain open

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Safety Warning

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.

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Possible Causes

Open high-limit switch, Open flue safety switch, Open burner door interlock, Loose wiring in safety loop

How to Fix / Troubleshooting

Safety: Turn off power. Never bypass safety switches to keep the furnace running.

Steps:

  • Check access panels: Ensure all Buderus burner and control doors are properly closed and latched. Some models have door interlock switches.
  • Inspect high-limit and flue switches: Locate the manual reset high-limit and flue safety switches. If a red or black reset button is present, press it after the unit has cooled.
  • Wiring check: Follow the low-voltage safety loop wiring and look for loose spade connectors or broken wires.

If O5 returns after resets and wiring checks, a safety device may be tripping due to a real fault (overheat, blocked flue) and should be diagnosed by a professional.

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Repair Difficulty

Hard 4/5

Required Part

Buderus High Limit Switch
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