This is exactly what the service manual says about the F 70 code word for word,
If a Central Control Unit Heartbeat Failure is displayed, no communication
is detected from the Central Control Unit (CCU) to the User Interface.
Check for continuity of the User Interface harness.
Check wire harness connections to the User Interface and the CCU.
If the drum is not rotating, unplug the MCU from the CCU
(the MI3 or M13 connector). If the code disappears, replace
the MCU. If the code does not disappear, replace the CCU.
Replace User Interface if drum is rotating.