BoilerFaultCodesWhat the manual says
Manufacturer manualAPC loadpump fault connected to F.91 - Inadequate heat transfer between the heater and storage tank. – Check storage tank charge sensor and storage tank sensor – Air in the APC pump – Check pump wiring harness – Secondary heat exchanger blocked – Priority changeover valve defective – Pump defective – Plate-type heat exchanger scaled
From this model's manual
Verified against the Vaillant/Vaillant Ecotec Plus 937 Vui Gb 376 3 5 Isim installation & service 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
All 38 fault codes on the ecoTEC plus 937
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