The service soon light means the car needs a service a lot of systems work out how many starts the vehicle has etc which will have an effect on the engine oil so it is important you do the service. The reluctor ring is on the driveshaft first thing is to make sure it is fitted correctly some cars have a seperate ring some are part of the CV Joint also make sure they are clean and the sensor is also clean, be careful with the sensor it is fragile piece and can be broken quite easy. Hope this helps.