Q. My Defender Td5 has started to misfire very badly. A diagnostic check didn’t find anything wrong. Is a sensor likely to be at fault, or could it be a more serious problem? Neil Long, Grimsby, Lincolnshire
A. Oil has probably got into the separate wiring harness that powers the injectors. This connects to the engine harness leading down to the ECU. Check at the ECU end by removing the connector and looking for oil.
You can clean it, but the most effective cure is to replace the injector harness. A little remaining oil may seep into the engine harness and could continue through to the ECU after replacing the injector harness, but the problem will nevertheless be cured. The part number of the harness is AMR6103; it costs about £30.