Note:
The reasons why the parking/additional heater is locked can be the following:
Under-voltage cutout.
Activation of the overheating protection.
Lack of fuel.
Test prerequisites:
No fault codes in the automatic climate control unit.
Connect the battery charger.
The fuel level should not be less than 15 liters.
Procedure:
Ignition on.
Remove the fuse for the parking/additional heater.
Wait for 5 seconds and then reassemble the fuse.
The blocking protection is now reset.
Start the parking/additional heater and let it be active for at least 3 minutes before shutting it off.