DIAGNOSIS The water is not reaching the correct brewing temperature. The primary cause is severe mineral scale buildup in the internal hot water tube or on the heating element.
FIX Run a full, strong vinegar descale cycle. Use a 1:1 ratio of white vinegar to water. Fill the reservoir. Place an empty carafe on the warmer. Run a full brew cycle. Let the vinegar solution sit in the unit for 30 minutes. Run two full cycles with clean water to rinse.
IF THAT DIDN'T WORK The thermostat or heating element may be failing. This requires component-level repair or replacement of the boiler assembly. The unit must be disassembled for diagnosis.
HEADS UP For maintenance, run a vinegar descale cycle monthly if you have hard water. The "Brew Strength" control only alters brew time, not water temperature.