991
Takagi Flue Blockage / Abnormal Combustion (High CO Risk)

Takagi water heater displays error 991; unit shuts down.

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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

Blocked exhaust vent, Blocked air intake, Incorrect vent installation, Severe combustion imbalance

How to Fix / Troubleshooting

Critical safety warning: Error 991 indicates a serious combustion or venting problem that can lead to carbon monoxide buildup. Turn OFF the heater immediately, shut OFF gas, and ventilate the area. Do not operate the unit until the issue is resolved.

Steps (homeowner level):

  • 1. Inspect vent termination: From outside, check that the exhaust and intake terminations are clear of snow, ice, leaves, nests, or other obstructions. Clear any blockage.
  • 2. Check venting for damage: Look for disconnected, crushed, or sagging vent sections. Do not operate the heater if venting is compromised.
  • 3. Professional inspection: Contact a licensed technician to perform a full combustion analysis, verify vent sizing and routing, and inspect the burner and heat exchanger. Components may need cleaning or replacement.
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Repair Difficulty

Professional Required 5/5

Required Part

Exhaust Vent System
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