2006 Citroen C4 anti pollution

Have you had it code read?
Apart from the coil pack everything else you have done is common sense,which,believe me isn't that common.
But you really need the code read,and probably live data read,
A pro will be able to tell from the live data where the fault is and possible causes,
otherwise your shooting in the dark trying to fix it by luck!

suggest you have a look at the exhaust mid section and tail pipe as sometimes a leak will cause the light to come on

I had my c4 serviced in July, a month later the engine management warning came on saying the anti-pollution was faulty. I took to a Citroen garage and they hooked it up to the computer. Apparently it was a faulty wire. This was fixed, but today it has come on again. The revs are very low and it seems to be misfiring because it's very jumpy and sounds like its coughing and spluttering as if it's choked up. I cant drive it very far, luckily the garage isn't far from where I live. Any ideas what may be wrong with it amd could this have shown as a warning when I took it in last month?