The front sensor is the important one as it signals to the ecu to alter fuel input to manage emissions. On the Gems system the rear one ( after the cat) is basically to check the cat is functioning properly. If emissions ( based on Oxygen level in the exhaust gas) are within limits then the dar should run ok. The system puts the MIL on if the rear one has problems. On my car the fault code shown was an open circuit in the downstream sensor heater circuit ( after cat) sensor. Basically the heater gets the sensor up to functioning temperature quicker than waiting for the exhaust to heat it.
Older systems did not have this heater but it was added later to get faster functioning to reduce emissions a bit more and quicker. A fine tuning method.
My OBD check tool gave the code P1193 which, for Rover/Land Rover is the downstream heater circuit open, hence not working. Car ran OK with no issues as the upstream ( front) sensors were OK.
