Land Rover Repair: check engine light, o2 sensors, output shaft

Question
I have a 98 discovery- the yellow check engine light is on the dealer says i need one oxygen sensor replace for $900- also the speedometer/odometer is not working. Any idea what is wrong ????  Thank You !

Answer
1 O2 sensor for $900?  I would get another opinion.  the O2 sensors for LR's are not cheap, ($100-$150) but should only take 1/2 hour to install, max.  

The spedometer comes off of the transfer case output shaft, it could be a problem with the sensor, the wiring, or the dash display itself.