
Checking and replacing the part prone to failure (less than £15) can save a bill of over £100 for the diagnostics and a new injector for the C3.
The 7th of March 2006 saw a Citroen Technical Bulletin being issued, warning workshops of the problem. Cars manufactured after this date should be exempt from the problem - look at this topic if you would like to see how to find the production date of your Citroen C3.
Improved design washer jets have Citroen part number (6438.V8 (here) and are now available. Fit two of the improved misting jets in place of your current jets if your car has not yet been modified and it was made before March 2006. If you are unsure if the modification has been carried out already, its cheaper to fit 2 new jets than it is to fit new injectors plus the diagnostics costs...

An upgraded washer jet to prevent injector trouble on the Citroen C3
There is a sealing ring added to the new design.
Find the upgraded washer jets for a C3 here.
It has been suggested that a similar solution to fitting modified washer jets (6438.V8) is to seal the original unmodified jets with some waterproof silicone sealant and this may prove to be true but for the cost of two upgraded washer jets it may not be worth opening a tube of silicone to protect the injectors.