The Catalytic Convertors are integral with what is called the 'Y-Pipe"-- one branch for each Bank of the V-8. Depending on your model, as many as four Oxygen (O2) sensors may be fitted. One upstream, and one downstream on each branch of the pipe. This is for the purpose of measuring Catalytic "efficiency" in the system . If a mapped threshold value in the ECU is not maintained, a Check Engine light will illuminate, and the fault code stored in the vehicle. Because the Pipe can be very pricey (as much as $2K from the Dealer) it is important that you confirm Catalytic Convertor failure before replacing the Pipe. O2 sensors do fail, but first check the wiring harness for damage-- at the sensors. It is usually necessary to have the system scanned by a professional shop to isolate which one has failed. Or-- you can "probe" the individual sensors at the connector to confirm the voltage present. Pins at the connector have been known to pull back. Oil/coolant contamination inside the connector can give erratic readings, so spray them with Electrical Contact Cleaner before re-connecting them--BUT-- check for the source of the contamination. We have seen more than a few Discovery models that had seeping head gaskets leak onto the wiring harness for the O2 sensor. The sensor fails, the ECU defaults to a rich setting (to protect the engine) on that bank, and it wipes out the Catalytic Convertor if driven too long under these conditions. Symptoms of a "packed cat(s)" are power loss, a burning "smell", and in dim light you will see them glowing cherry red.
Part of the emission control system implements a purge valve. The Range Rover P38 and Discovery I seem to be more prone to failure here. You may experience a situation where the vehicle runs fine when cold, but then misfires, or will not pull above a certain speed when warm. So-- before diagnosing the problem as a packed-in Catalytic Convertor- remove the Gas Cap-listening for a strong sound of rushing air at the filler neck (not merely a "hiss" for a second after loosening the cap). The fuel pump is immersed in fuel as it resides in the gas tank. The failure of the purge valve results in a strong vacuum in the tank, and puts a strain on the pump--hence--- improper fuel volume making its way to the engine. This also greatly shortens the life of the pump.
