What does the Vaillant d.66 fault code mean?

This code simply confirms whether your boiler is currently set to keep a small amount of water pre-heated for faster hot water taps, rather than indicating a fault.

How much does it cost to fix Vaillant error code d.66?

Cost varies. Contact a Gas Safe registered engineer for a quote.

Vaillant d.66 Fault Code: What It Means & How to Fix It

What the manual says

Manufacturer manual

Activation of the warm start function for hot water: off=Function deactivated, on=Function activated

Shared across the range

This code is shared by the boilers in this range, which use the same control board. It may not be printed in this exact variant’s manual.

DIY fix possible

Locate the menu button on your boiler displayNavigate to the 'Comfort' or 'Warmstart' settingSelect 'on' if you want faster hot water, or 'off' to save on energy costs

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All 170 fault codes on the ecoTEC Plus 635

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

con d.00 D.000 d.01 d.02 d.04 d.05 d.06 d.07 d.08 d.09 d.10 d.11 d.13 d.14 d.15 d.16 d.17 d.18 d.19 d.20 d.21 d.22 d.23 d.24 d.25 d.27 d.28 d.31 d.33 d.34 d.35 d.36 d.39 d.40 d.41 d.43 d.45 d.47 d.50 d.51 d.58 d.60 d.61 d.62 d.64 d.65 d.67 d.68 d.69 d.70 F.00 F.0 F.01 F.1 F.10 F.11 F.12 F.13 F.20 F.22 F.23 F.24 F.25 F.27 F.28 F.29 F.32 F.33 F.49 F.61 F.62 F.63 F.64 F.65 F.67 F.68 F.70 F.71 F.72 F.73 F.74 F.75 F.76 F.77 F.78 F.83 F.84 F.85 F.86 F.87 F.88 F.89 F.91 F.92 F.97 F22 F70 P.00 P.0 P.01 P.1 P.02 P.2 P.3 P.4 P.5 P.06 P.6 P.07 S.0 S.01 S.1 S.02 S.2 S.03 S.3 S.04 S.4 S.05 S.5 S.06 S.6 S.07 S.7 S.08 S.8 S.10 S.11 S.13 S.14 S.15 S.16 S.17 S.20 S.21 S.22 S.23 S.24 S.25 S.26 S.27 S.28 S.30 S.31 S.32 S.33 S.34 S.36 S.39 S.41 S.42 S.46 S.53 S.54 S.88 S.91 S.96 S.97 S.98 S.99 S.108 S.109 T.02 T.03 T.04 T.05 T.06 T.08

This code simply confirms whether your boiler is currently set to keep a small amount of water pre-heated for faster hot water taps, rather than indicating a fault.

What the manual says

DIY fix possible

Avoid the next call-out bill

All 170 fault codes on the ecoTEC Plus 635

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