Mercedes-Benz vehicles such as E-Class W211 and CLS-Class W218 may stop blowing hot or warm air even when the climate control settings is set to maximum heat. This problem is often caused by a defective Mercedes heater control valve. To fix the problem you will need to remove the heater control valve which is located in the engine bay by the firewall.
Open hood by pulling the hood release under the dashboard.
Remove the three 13 mm nuts that hold the windshield wipers in place. Remove the wiper arms by pulling them up.
Next, you need to remove the plastic trim. Before you do so, you need to remove the four small plastic screws. Remove the plastic trim by pulling it up.
Remove the windshield wiper linkage. There are two 13 mm nuts and one 13 mm bolt that needs to be removed. Once the linkage bolts are removed, move the linkage assembly to the side.
Now you can access the heater control valve.
Use needle nose pliers to pull on the retainer clips. You don't need to remove the clips completely. They need to be pulled out slightly. This will allow you to remove the hoses. Note that coolant may come out of the hoses.
Remove the clamp on the coolant hose on the left side of the heater valve.
Disconnect the electrical connector. Remove the old heater valve and install a new one in reverse order.