BMW models that have the IHKA w/MAX button:

...this feature operates at ambient temps between 41°F-up...and if the ambient temp is above 61°F...the fresh air flaps close and the fan speed goes to the maximum velocity. As others mentioned...it also still has the original REST (residual heat feature) that can be initiated when ambient temps are BELOW 59° F.

BMW models that have the IHKA w/REST button:

...this feature allows you to use the accumulated heat on cool/cold days to keep the interior cabin warm while running errands or if stopped at a railway crossing (you can turn the engine off).

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