High
F.023 on Vaillant ecoTEC plus 940 VUI 30/40CS/1-5

Your boiler has detected that the water leaving it is much hotter than the water returning, suggesting a blockage or a problem with the pump circulating the water efficiently.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

What the manual says

Manufacturer manual

The temperature spread between the flow/return is too great.

From this model's manual
Verified against the Vaillant/Vaillant Vaillantpro 0020308121 05 Ecotecplus Storage Combi Installation installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F.22View →F.24View →F.75View →

All 125 fault codes on the ecoTEC plus 940 VUI 30/40CS/1-5

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

194F.000F.001F.002F.003F.010F.011F.012F.013F.020F.022F.024F.025F.027F.028F.029F.032F.035F.040F.042F.044F.047F.049F.057F.061F.062F.063F.064F.065F.067F.068F.070F.071F.072F.074F.075F.076F.077F.078F.080F.081F.083F.084F.085F.087F.088F.089F.090F.092F.095F.096F.097F.105F.194F.195F.196F.317F.318F.320F.321F.322F.323F.324F.325F.326F.327F.330F.334F.336F.337F.338F.342F.343F.344F.347F.363F.707F.905I.003I.020I.144L.016L.022L.025L.032L.095L.096L.097L.105L.144L.194L.195L.196L.319L.320L.322L.343N.013N.027N.032N.089N.095N.096N.097N.100N.144N.194N.270N.317N.319N.324N.343P.000P.001P.003P.008P.021T.001T.002T.003T.004T.005T.006T.007

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