I have a 01 Terrano and the ABS sensor lights keeps coming on and off, the computer is telling the mechanic a G sensor but he doesn,t no what that is? Any help on this? also when the steering is locked round hard there is a lot of noise and clanging coming from the suspension

The ABS sensors on my Terrano are a magnetic pick-up, which senses the passing of the teeth machined into a steel disk on each wheel.
If the ABS computer codes are indicating the G sensor, then that is probably what is at fault. The G force sensor is usually located between the front seats, under the rear of the center console (probably a metal box a little larger than a box of matches). The purpose of the G sensor, is to sense how slippery the road surface is. If you brake on good sealed road, the deceleration G forces are much higher, than if you brake on loose gravel - even if all four wheels were to lock up, the G force sensor still registers the vehicles movement, and tells the computer to ease off on the brakes, as the vehicle is still moving, despite all four wheels having stopped.

You have one of them toothed wheels loose on one of the wheels that the abs sensor reads .

Abs sensors work by an electric eye ( similar to the chime at a shop door) shining on a toothed "gear" on each wheelhub, these detect which wheels are turning and which are locked. Dust and mud can get into plugs and more rarely cake around the teeth on the "gears" .
If you are getting noises from the front end on lock something may be seriously worn or damaged in the hub.
First step is to isolate where the noise is coming from then dismantle and investigate. Nissans have notoriously dodgy wiring so make sure it is all clean and in good condition after fixing the clanging.