F20 or Heating fault on Miele affects heating. The Thermostop wait can pause the timer until the temperature is reached, but a persistent error needs inspection.
Timer stopped or fault displayed
The manufacturer documentation explains that Thermostop keeps the wait active when the program temperature has not yet been reached; the time starts counting down again once it is reached. Note whether only the timer stops or F20/Heating fault appears explicitly.
Record the program, selected temperature, and the stage at which it stops. The feel of the glass or the final temperature of the clothes alone does not measure how the water was heated during the wash.
A limited reset
For the technical fault, the support consulted says to disconnect from power, wait at least two minutes, and then reselect the program. Do this only with dry, safe access to electricity, and confirm that it matches your model's manual.
If the warning returns, stop retrying and request service. Do not switch to cold washes to keep using an appliance with a persistent error, and do not make internal measurements or bypasses on thermostats. The guide to electrical and heating helps describe the related symptoms.
The diagnosis must confirm the cause of the heating problem; F20 does not instruct you to replace a specific heating element.