My 2003 Mondeo TDCi "dropped the code" on one of its injectors around Christmas, and although my mechanic (a diesel specialist but not a Ford dealer) re-programmed it so that it drove well again, he did not replace it. The problem has now re-occurred, but he now recommends going to Ford to have it replaced. Is it OK to replace just one faulty injector, or should all 4 be done together? Doing all 4 would obviously be a better option, but also 4 times the cost, so not a cheap job! Thanks