Peugeot 307 ABS sensor warning?

Our 307 comes up with a warning of an ABS fault in the central display. This shows when you start the engine before the car even moves. The dealer said the diagnostics show it as a rear wheel sensor needing replacement.
I got another sensor and plugged it in to the plug that the old sensor was plugged into (without fitting it completely). It still shows an ABS fault before the car moves.
Is there a way to test the wiring in case a wire has chafed? What signal should I see at the plug and how can I check the sensor? ie is there a short or a fixed resistance between the wires. I can't find out what is in the sensor.
Thanks
David