C20
Bosch EasyControl
Modulation Control Limitation / Degraded Mode
Bosch EasyControl shows error C20 and indicates that the boiler modulation control is limited or disabled.
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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
Boiler not responding to modulation commands, Boiler set to fixed flow temperature, Incompatible modulation protocol, Faulty modulation output on EasyControl
How to Fix / Troubleshooting
Safety first: Isolate power before working on boiler controls.
- 1. Check boiler settings: In the boiler installer menu, ensure that modulation via OpenTherm/EMS is enabled and that fixed temperature mode is disabled if you want full modulation.
- 2. Observe boiler behavior: During a heating demand, watch the boiler’s display to see if it reports modulation percentage or flow temperature changes commanded by EasyControl.
- 3. Reset to defaults: Reset boiler control parameters to factory defaults if they have been heavily customized, then re-enable external control.
Professional help: If C20 persists and the boiler ignores modulation commands, a heating engineer should test the boiler’s control board and the EasyControl interface.
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Repair Difficulty
Medium
3/5
Required Part
Boiler modulation control board
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