Sensor under airfilter

Hi guys theres a sensor cable which is located under the air filter of the Citroen c3 1.4 hdi.my mechanic disconnected it now im not sure where or what it connects to please help very urgent.

Hi

Please ask your mechanic to refit the connector.

It's not possible to know what the connector is for without a symptom of a problem or a picture of the connector in context of its location.

How do you know the mechanic disconnected it?

Flexbruv, is your engine a diesel or petrol version? Your bio says petrol but you have posted in the diesel area. It might be relevant to know which engine you have.

There are quite often connectors in the looms that are used on other models or variants but not on yours. If the unused connector is in the engine bay it should be "blanked off". That is, it should have a plastic cap on it. HOWEVER people tend to fiddle with stuff and don't put things back the way they found it.
If its a white connector and your engine is a petrol engine then the connector doesn't go anywhere. I can't comment on a diesel engine as I am not familiar with them.
If the connector is a different colour, can you take a photo and post it here.
If your engine is otherwise running fine, you can assume the connector is not used on your car.

Does it look like this?