A quick look for O2 sensors on the web for an 87 SIII XJ6 yields many results -- even from the same vendor.

I believe the best O2 sensor for this model would be a single wire Bosch unit but I am not sure what is the p/n. Wondering if anyone has a p/n they can recommend?

 

Your '87 uses the three-wire oxygen sensor with the heater element. A quick check of Amazon shows that an OE Bosch part number would be 13032, but really these things are all pretty generic; you can get them dirt cheap with bare wires and "OE" just means the connectors are included for your car. I just replaced mine with a Delphi which is good quality and fits fine.
Best,
Andrew.

 

I seen web references claiming the later 85-87 SIII XJ6 are 3 wire O2 units from the factory will others say all 79-87 SIII are single wire.

My 87 US Market XJ6 has a single wire O2 sensor on it now but I can't say for sure if it came with a 1 wire or 3 wire.

Anyone have a factory SIII that they know for sure came with a 3 wire O2 sensor?

 

My California '87 came from the factory with a 3 wire O2 sensor. My '86 came from the factory with a single wire unit that was replaced by a 3 wire unit. Jaguar introduced a 3 wire conversion unit for '86 that was installed by the dealer as part of an upgrade/recall during servicing. Found a conversion kit at a local Jaguar parts supplier here in California recently and bought it as a spare for less than the cost of the o2 sensor.

 

This could explain the listings for both 1 wire and 3 wire sensors for the later years.

As I understand it, the three wire sensor just adds a heating element to warm up the sensor faster so that the system can operate in closed loop feedback mode sooner.

Interesting thing is that I notice my car would idle smoother when cold and not so well when warm. I always assumed that was because the mixture would lean out some when the engine warmed up and the operation was switched to closed loop mode.

Maybe I'll stick with the one wire sensor to let it run a little richer and smoother a little longer.