What does the Viessmann C4 fault code mean?

Your boiler has lost its connection to a separate heating control unit, such as the room thermostat or a weather controller, so it may not be following its usual settings, but it is not a safety hazard.

How much does it cost to fix Viessmann error code C4?

Typical repair cost for the Viessmann C4 fault is £80-200 including labour.

Viessmann C4 Fault Code: What It Means & How to Fix It

What the manual says

Manufacturer manual

Communication fault, OpenTherm extension

From this model's manual

Verified against the Viessmann/Viessmann Vitodens 200 W B2ha B2ka 5772909 installation & service manual.

DIY fix possible

Turn off the boiler at its on/off switch and at the fused spur or isolator, then wait two minutes.Check that the wires on the room thermostat or external control unit are fully pushed into their terminals and look undamaged, without opening the boiler casing.Restore power, then press and hold the reset button on the boiler (or use the reset option on the display) and check whether the C4 code clears.

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Related fault codes

C5View →F0View →

All 83 fault codes on the Vitodens 200-W B2KA-35

Seeing a different code? Every code this model can display, in one place.

4C 9A 9b 9C 9E 9F 10 18 19 20 28 30 38 40 44 48 50 51 58 59 90 91 92 93 94 98 99 A3 A7 b0 b1 b5 b7 b8 bA bb bC bd bE bF C1 C2 C3 C5 C6 C7 C8 Cd CF d6 d7 d8 dA db dC dd dE dF E0 E1 E3 E4 E5 E7 E8 EA Eb EC Ed EE EF F0 F1 F2 F3 F8 F9 FA FC Fd FE FF

Your boiler has lost its connection to a separate heating control unit, such as the room thermostat or a weather controller, so it may not be following its usual settings, but it is not a safety hazard.

What the manual says

DIY fix possible

Avoid the next call-out bill

Related fault codes

All 83 fault codes on the Vitodens 200-W B2KA-35

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