Controlling 4pin fan with PWM 25Khz

This works very well for an application that I'm building, but...how is it that I can't use another pin assignment (pin 5, for example) just by changing the VentPin declaration? I've tried it and there's no PWM output on that pin showing on the scope; is this coupled to the timer routine in a way I'm not seeing?

dlloyd:
Yes, 25kHz PWM is possible ...

word VentPin = 3;

void setup() {
  pinMode(VentPin, OUTPUT);
  pwm25kHzBegin();
}

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