Your boiler is currently refilling its internal water pressure system automatically, which is a normal process intended to keep your heating running smoothly.

DIY fix possible

Wait for a few minutes while the boiler completes the automatic repressurisation cycle.Observe the pressure gauge on the front of the boiler to ensure it settles within the green zone (typically 1.0 to 1.5 bar).Once the cycle is complete, the S.99 code should disappear and the boiler will return to its normal standby or heating mode.