Or, rather than an OBD or OBD II reader, go get the electronic service manual via the owners club or on the web.
It's saved me a fortune !
My Primera is loaded with gadgets and all the controls are from one screen, but it's biggest issue is it's not used much, or just short journies now. It's often low on fuel, so having it low, picks up crap from the petrol tank (the car is 9 years old now) and causes an occasional shudder. This then fires up the engine management light. The cure is to disconnect the battery over night (needs to be about 12 hours - 3 or 4 doesn't work), this clears the ECU memory and no faults are then picked up. Sorted.
If it came on again, then you do have a fault.
Mine just does this because it's low on fuel. I also run some fuel system cleaner through - drop some in the petrol tank on fill up. This then ensures it runs well for some time - or until I let it run too low.
Last week I had an airbag warning come on. Wouldn't go off. Quickly searched on line and there was a method, without using a computer, to tell the system to run a system check. A few on/offs with the ignition at the right time, and the system cycled through, but started to flash. A fiddle with the wires under the driver's seat, and next to the side air bag - bingo, up flashed the warning again (kids had kicked a wire loose). Re-ran the on/off ignition cycle, and the system came up clear.
So get an electronic servcie manual - at least you'll know what the error is before some garage spends weeks changing out sensors.