What does the Vaillant D.028 fault code mean?

Your boiler has detected an issue with an external device connected to it, such as an extra pump, a diverter valve, or a smart heating controller.

How much does it cost to fix Vaillant error code D.028?

Typical repair cost for the Vaillant D.028 fault is £120-250 including labour.

Vaillant D.028 Fault Code: What It Means & How to Fix It

What the manual says

Manufacturer manual

Functionofext.accessoryrelay2

From this model's manual

Verified against the Vaillant/Vaillant Ecotec Plus installation & service manual, page 74.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer

Search the official register →

Boiler cover options

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Related fault codes

D.027View →F.76View →

All 281 fault codes on the ecoTEC plus System

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

con D.000 D.001 D.002 D.003 D.004 D.005 D.006 D.008 D.009 D.010 D.011 D.012 D.013 D.015 D.016 D.017 D.018 D.020 D.021 D.022 D.023 D.025 D.026 D.027 D.029 D.031 D.033 D.034 D.035 D.036 D.039 D.040 D.041 D.043 D.045 D.047 D.052 D.058 D.060 D.061 D.062 D.064 D.065 D.066 D.067 D.068 D.069 D.070 D.071 D.072 D.073 D.074 D.075 D.077 D.078 D.080 D.081 D.082 D.083 D.084 D.085 D.088 D.090 D.091 D.092 D.093 D.094 D.095 D.096 D.098 D.124 D.125 D.128 D.129 D.145 D.146 D.147 D.156 D.157 D.158 D.159 D.160 D.161 D.162 D.163 D.164 D.166 D.167 D.170 D.171 D.172 D.173 D.174 D.175 D.182 D.185 D.186 D.187 F.000 F.001 F.002 F.003 F.010 F.011 F.012 F.013 F.020 F.022 F.023 F.024 F.025 F.027 F.028 F.029 F.032 F.035 F.040 F.042 F.044 F.047 F.049 F.057 F.061 F.062 F.063 F.064 F.065 F.067 F.068 F.070 F.071 F.072 F.074 F.075 F.076 F.077 F.078 F.080 F.081 F.083 F.084 F.085 F.087 F.088 F.089 F.090 F.092 F.095 F.096 F.097 F.105 F.194 F.195 F.196 F.317 F.318 F.320 F.321 F.322 F.323 F.324 F.325 F.326 F.327 F.330 F.334 F.336 F.337 F.338 F.342 F.343 F.344 F.347 F.363 F.707 F.905 I.003 I.020 I.144 L.016 L.022 L.025 L.032 L.095 L.096 L.097 L.105 L.144 L.194 L.195 L.196 L.319 L.320 L.322 L.343 N.013 N.027 N.032 N.089 N.095 N.096 N.097 N.100 N.144 N.194 N.270 N.317 N.319 N.324 N.343 P.000 P.2 P.008 S.0 S.000 S.1 S.001 S.2 S.002 S.003 S.4 S.004 S.005 S.006 S.7 S.007 S.008 S.009 S.10 S.010 S.11 S.011 S.012 S.013 S.014 S.15 S.015 S.016 S.17 S.017 S.019 S.20 S.020 S.021 S.022 S.023 S.024 S.025 S.026 S.027 S.028 S.029 S.030 S.031 S.032 S.034 S.039 S.041 S.042 S.054 S.057 S.059 S.088 S.091 S.092 S.093 S.096 S.097 S.098 S.109 S.175 S.199 S.326 S.328 S.335 S.341 S.599 T.001 T.007

Related guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on combis.

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Banging / kettling / whistling noise

Each noise points to a different fault.

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Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

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Your boiler has detected an issue with an external device connected to it, such as an extra pump, a diverter valve, or a smart heating controller.

What the manual says

Engineer required

Avoid the next call-out bill

Related fault codes

All 281 fault codes on the ecoTEC plus System

Related guides you might find useful

No hot water but heating works

Banging / kettling / whistling noise

Yellow or orange flame — CO safety

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