JBraak

I have a 2012 Grand Cherokee CRD Overland. The car has done 156000 km and since 80000 km my engine warning light is on. The dealer informed me the problem is with cylinder #4 and the injector needs replacement. This I do not believe as the car has no lack in power, the fuel consumption is good and engine runs smooth. I just keep driving the car with this light on. With my last service I asked the dealer to swop injector 4 with another cylinder e.g cylinder 2. If the injector is faulty, the fault will now show on cylinder #2. However, after the swop the error is still with cylinder #4. This means the injector is not faulty. The dealer does not know what the problem is. Did anyone experience similar issues before? Any advice how to find the actual error?