It is also vitally important to check the coolant level. To check it, you have to remove the radiator cap from the engine. A word of caution; always check it when the radiator is cold and when the engine is not running.
If the coolant level is low, the resistance will not be able to heat all parts of the engine correctly. In fact, it could even burn the resistance itself, as it is not in contact with liquid.
You must top up the radiator with suitable coolant, but never with water. Likewise, it is important to notify the technical service team if you detect any leakage or any spotting from a possible coolant leak.