How can I cancel the low water pressure warning lght on my Puma 80/100 com ?

The low water pressure warning light has activated on my Puma 80/100 combi boiler, thus preventing by boiler from `firing up'. The water pressure dial shows the water pressure as OK and I have increased the pressure (within safety limits as indicated on the dial) in an effort to cancel the low pressure warning light. The warning light remains on. How can I rectify, so the warning light is deactivated and my boiler can operate normally?