2000 Citroen Xsara RPM Unstable

I am not comfortable with the engine sound, its rpm rough running and fuel consumption. I had it inspected by a local workshop and after tuning it, I was told that I had to replace the engine speed sensor and the multifunction switch. I do not know where these are and what it does as I am not familiar with new cars DIY. The two lights S and * next to the gear position indicator are not even blinking to signal defects. Only if I press it, will it light. Can you please tell me what these items are as I do not want o buy these if it has no bearing to the engine and its consumption. Where are these located in the engine or gearbox? How it can it be tested if indeed these are parts for replacement?

Also the cooling fan is switching to higher speeds and low intermittently even if the temperature is low. Its not like the fan is at high speed continuously even at low temperature. The fan goes high then low rather intermittently even if I had jsut started and even without the A/C on. What can be the problem? How can it be diagnosed?

Thanks and with kind regards,

John David