I had a permanent ECU light with code 94 stored. I bought a sensor last week and replaced it at the weekend. I reset the ECU and then started it - no light, no fault codes. The car is running a lot better than it was and the C20LET manual states that the Hall Sensor affects fuel consumption and cold starts. The car still has a hesitant type misfire at high revs but I've still to try replacing the HT leads.
I had a look at the old hall sensor and noticed that there were little bits of metal stuck to the magnetic points of the sensor. I found that these bits of metal had been parts of the slide on clip that holds the plug for the hall sensor in place - the one on the side of the distributor. Mine is zip tied in place because this clip has pinged off inside at some point. After cleaning the sensor up it looks fine and I'm convinced that there is nothing wrong with it and that it was these bits of metal blocking the sensor that caused the fault code. So now I have a perfectly good spare hall sensor (99% sure anyway!).
So the moral of the story is, take a look at your old hall sensor if you have an ECU light and clean it up before buying a new part!
Thanks to Strick for supplying the part number for the hall sensor