A4
Viessmann
Indoor Heat Exchanger Overheat / Protection
Indoor unit stops; A4 displayed; unit may short-cycle in heating mode (for heat pump models)
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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 indoor air filter, Obstructed air inlet or outlet, Faulty indoor fan motor, Incorrectly installed or detached coil sensor
How to Fix / Troubleshooting
Safety: Turn off power before removing covers.
- Clean filters: Remove and wash the air filters with mild detergent and water. Dry completely before reinstalling.
- Check airflow: Ensure no furniture, curtains, or covers block the front intake or discharge louvers.
- Inspect fan operation: With the cover off and power restored, briefly run the unit and confirm the indoor fan starts smoothly at all speeds. If the fan stalls, runs slowly, or makes grinding noises, the fan motor or fan capacitor may be failing.
- Verify coil sensor position: Confirm the evaporator/heat exchanger sensor is firmly attached to the coil. A loose sensor can misread temperature and trigger overheat protection.
- Reset and test: After cleaning and checks, reset power and test in the affected mode (cooling or heating).
If A4 persists with good airflow and a correctly mounted sensor, the indoor fan motor or control PCB may require replacement.
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Repair Difficulty
Medium
3/5
Required Part
Indoor Fan Motor
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