DIAGNOSIS The machine's thermostat or heating element is failing, or mineral scale is insulating the heater.
FIX First, ensure you are using the 'Strong' brew setting. It uses a slower flow rate for hotter extraction. If already on 'Strong', run multiple cycles without a K-Cup to heat the internal components thoroughly. If the water remains lukewarm, descale the machine immediately.
IF THAT DIDN'T WORK The thermal sensor or heating element is defective. This requires professional repair. Contact Keurig support.
HEADS UP Preheat your mug with hot water before brewing. Glass and ceramic mugs draw significant heat from the brew, causing a noticeable temperature drop.