Problem with Step Tunning at line 244 "calculate the standard deviation"

[succZS]

I really like this PID Controller, but I have one issue.

When I try to use the Step Tuning feature, I always get 0 or close to 0 as the calculated ioKp. I have a really stable input at the start.

I think it's because of these lines of code:

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REGION Step tuning
CASE #statStepTuning.state OF
0:
REGION Initialize tuning
// Init tuning variables
#statStepTuning.find_u_max := #temp_uF; // Let's say I have a 20.0°C input, so #statStepTuning.find_u_max = 20.0
#statStepTuning.find_u_min := #temp_uF; // Let's say I have a 20.0°C input, so #statStepTuning.find_u_min = 20.0
#statStepTuning.calc_u_dev := 0.0;
#statStepTuning.calc_u_start := 0.0;
#statStepTuning.find_k_inflection := 0.0;
#statStepTuning.find_u_inflection := 0.0;
#statStepTuning.find_t_inflection := 0.0;
#statStepTuning.calc_m := 0.0;
#statStepTuning.calc_L := 0.0;
#statStepTuning.calc_Kc := 0.0;
#statStepTuning.calc_Ti := 0.0;
#statStepTuning.calc_Td := 0.0;
#statStepTuning.t := 5.0;

            // Transition
            #statStepTuning.state := 1;
        END_REGION
        
    1:
        REGION Calculate the standard deviation
            // Find min and max input
            IF #temp_uF > #statStepTuning.find_u_max THEN   // This condition becomes (IF 20.0 > 20.0 THEN...), which is false.
                #statStepTuning.find_u_max := #temp_uF;
            END_IF;
            IF #temp_uF < #statStepTuning.find_u_min THEN   // This condition becomes (IF 20.0 < 20.0 THEN...), which is also false.
                #statStepTuning.find_u_min := #temp_uF;
            END_IF;
            
            // Calculate the standard deviation
            #statStepTuning.calc_u_dev := #statStepTuning.find_u_max - #statStepTuning.find_u_min; // This calculation is (20.0 - 20.0), resulting in 0.0.
            
            // Transition
            // Timer runs out / calculate starting point
            IF #statStepTuning.t <= 0.0 THEN
                #statStepTuning.t := 0.0;
                
                // Adjust derivative filter to corresponding deviation
                #statAdvanced.derivateFilter := #statStepTuning.calc_u_dev * 40.0; // This becomes (0.0 * 40.0), which basically disables the derivative filter.
                #statAdvanced.derivateFilter := #statStepTuning.calc_u_dev * 40.0; // This becomes (0.0 * 40.0), which basically disables the derivative filter.
                // Save starting point
                #statStepTuning.calc_u_start := #statStepTuning.find_u_min + #statStepTuning.calc_u_dev / 2; //This becomes (20.0 + 0.0) /2 
                #statStepTuning.find_k_inflection := #"temp_du/dt"; // Save slope
                
                // Save start output
                #statStepTuning.find_y_start := #statOut;
                
                // Increase output to make a step
                #statOut := #statAdvanced.outMax;
                
                // Change state
                #statStepTuning.state := 2;
            ELSE
                // Decrease step timer
                #statStepTuning.t := #statStepTuning.t - #temp_dt;
            END_IF;

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If I am wrong, feel free to correct me.