When an O2 sensor goes bad, the computer goes into a default mode since it no longer gets the live info regarding if the engine is running rich or lean. Unfortunately, Rovers use a much less common type of sensor called a Titania sensor as opposed to the normal Zirconia sensor.
Just found this website though, best prices I have ever found on a Titania type sensor. This is for a 94/95 truck. A 97 has double the number of sensors and they are a bit pricier per each.
Get the universal, the only difference is the wiring connectors and you can splice it right in and save yourself a bunch of money.