Sauna experiment - Celsius to Fahrenheit converter
Review

Several issues have also been addressed in the comments below your question. A quick re-iteration:


You state the user only knows Fahrenheit, yet you ask them to input in Celcius: Enter a desired temperature between 73 och 77 celcius.
For each bad attempt, you add the new input to the previous input temperature = temperature + celcius;. This doesn't make any sense at all.
You should be able to read from code what the code is about. tempCheck(double temperature) doesn't tell us the unit of the input. Prefer celcius or a custom class Celcius.




tempCheck(double temperature)

This method has two very different responsibilities: (1) detect status by provided temperature (2) render status information to user. These should be two different methods to allow for reusability of the status check.

The status could be an enum:

enum TemperatureStatus
{
    Normal,   // 73°C - 77°C (but not 75°C)
    Perfect,  // 75°C
    Freezing, // &lt; 0°C

    // and so on ..
}


Have tempCheck return the status:

static TemperatureStatus tempCheck(double celcius)
{
    if (celcius &lt; 0) return TemperatureStatus.Freezing;

    // and so on ..

    return TemperatureStatus.Normal;
}


Rendering the status message can then go to its own method:

static string RenderStatus(TemperatureStatus status, double celcius)
{
    celcius = Math.Round(celcius);
    switch (status)
    {
        case TemperatureStatus.Freezing:
            return "The sauna has now become a freezer, \nCelcius: {celcius}";
        // and so on ..  
    }
}