The sensors are reading AFR >< 14.7, the AFR should be > at cold start so the voltage shoudl be near 1.0. Once warmed up the ECU functions to cycle the AFR so that the sensor voltage cycles between 1.0 and 0.1 - more or less with some variation. If they are cycling everything is normal. If it reads near one at startup, they likely are functioning correctly and any other readings are likely caused by other engine issues. If they don't start around 1 or never cycle - they are likely dead. Once you find one that cycles, treat it as a good one and use it as your control unit in swap experiments. If it cycles in both locations, but the other one does not, the other one is bad. If it cycles only in one location, you have a AFR issue or wiring issue on the side it will not cycle on.

 

So I want both left and right to start off around 1.0 then cycle? Starting at 1.0 then dropping to .5 and staying there without fluctuation means it's dead? have two that will cycle and both sides have cycled but i don''t remember which did what where

Does it matter that I have been testing them with the ignition key in the second position instead of starting the engine? i have only started it when both sides were reading something.

i still don't understand what would cause one of them to start off at .9, drop to .5, then slowly go down to zero and stay there.

The wiring diagram is very helpful, by the way,

 

I do need a spare Disco and nearly bought a 2001 for $500 last week. The owner had disassembled the engine to do the head gaskets but was killed in a car accident two weeks ago. His buddy is selling the Rover but knows nothing about it's history--or where the keys are.

 

Yes

I am not sure the exact cause of that, but in my experience it has been bad sensors.



Yes, they will only cycle with the engine running. without the engine running they should read 0.

The ECU controls the cycling by giving the engine more fuel until is sees a reading close to 1 v, then less fuel until it sees approx. 1 volt, then repeats the cycle. Occasionally it will throw a test in there that changes the cycle for other purposes (SAI for example) and will defuel the engine (0 volts) on long deceleration (like down a hill with foot off gas for several seconds).

When the engine starts cold the ECU gives a rich mixture (1 volt), as it warms up it lowers the fuel until it reads .1 volts and then starts cycling. If it starts at 1 volt and then goes to .1 volt and stays there, it could be a lean running bank due to vacuum leaks or bad MAF. Best to check that with a known good 02 (cycles) and if the known good 02 does not cycle, you have an engine issue not a sensor issue.

Swap them around until you get two that are cycling. Use those as control, swap the others in to see if they cycle. If you know the bank is cycling, but one of the sensors does not on that bank, you know it is bad.

 

$400-500 is a fair price if the body and interior are good, I would probably have bought it.

 

Also, the four wires going to the sensor, two are the heater power and ground (12v+ and ground) and two are the ref. ground and sensor output wires going to the ECU. The heater wires have no effect on the readings in the ECU, except the sensor will die relatively quickly without the heaters. You will get a code with no heaters, so they are apparently all fine.

 

 

#1 killer of O2 sensors is blown head gaskets leaking glycol in to exhaust.