?? injector no4 problem

Hi guys, my m8 has just purchased a 300tdi AUTO disco, it's running like a bag of @#$@, timing was out which has now been sorted but he keeps having an intermittent fault which flashes up the check engine light, I'm 99% due it's injector no4 (the one with the wire on it) the wiring has come apart at some time and been repaired, the question is can I test to make 100% sure it is this injector as £225 is a lot of money for me to have a guess at, can you plug an odb reader into these? I have very little experience with the 300tdi auto so all advise would be welcome