what is a engine speed sensor on a citreon c2 1.6 vts,and what does it look like

Their usually by the transmission or right on it and look something like a oxygen sensor with 2 wires but there would be two of these kind of sensors one being input other being output.Basically the sensor detects the speed and sends different levels of voltage to the dash speed gauge therefore it gives you your exact estimated speed.

Hope that helps and best of luck.By the way a good Haynes service manual should tell you the local and and how to trouble it too.