[D2]

The gearbox speed sensor is changeable but I've never seen one go faulty I'm I'm being honest! It's usually more likely to be an aftermarket wired in kmh to mph convertor that's went faulty, I'd check that and the wiring around it first

 

[D2]

If your car is a FQ model it should have a plug in type convertor between the speed sensor and original wiring plug, easiest way to find out if it's the converter is faulty would be remove the convertor and plug the original plug straight into the sensor and take it for a drive

 

[D2]

Glad you got the issue sorted mate