The early Citroen C3 (2002-2003, L4 level engine) has a vacuum controlled EGR valve.
The test for this is to remove it from the car and attach a hand held vacuum tester like this.
Observe the valve open with vacuum applied, the valve should hold in the open position and stay open until the vacuum is released.
If the valve won't open with vacuum then it may be stuck and need cleaning. Clean and test again.
If the valve won't stay open under vacuum it is faulty and needs a replacement.
If the valve won't close after it opens and vacuum is released then it may be dirty, clean it and test again.
If testing fails after cleaning, replace the EGR valve, part number 1628.XV
If the factory clip was removed to release the EGR valve from the EGR pipe, fit a replacement EGR clip, part number is 1628.3J (here) 'Tube Collar'.