The terms EMS (engine management system) and ECM ( engine control module) are one and the same
It's easy if you understand how the data systems work
The ECM receives the ambient temperature from the instrument pack via Can-bus it also sends it to the HEVAC.
Because of the failure of the Can-bus controller in the EMS the data packet sent by the instrument pack gets no response from the EMS and can no longer see the ambient temperature as being valid.
Because the HEVAC then also doesn't get a valid ambient temperature signal it won't let the HRW work.
It will also effect the ambient temperature signal on the K-bus to the BCU also as the instrument pack converts Can-bus to K-bus, the DS2 bus as well as the window bus and Nav-bus (if fitted)
Can-bus data always takes priority over any other data bus.
Russ