No such thing as a stupid question, sorry for assuming that everybody knows the difference.
They are the different engine management systems.
http://www.roverparts.com/TechTips/boschvsgems.cfm

 

Sorry, but you've got that switched around. The older GEMS system uses 2 (two) sensors and they are located before the catalytic converters. The Bosch system uses 4 (four) 2 before and 2 after the catalytic converters. In general Disco 1 trucks have 2, and Disco 2 trucks have 4.

 

Thanks for the link. I'm slowly learning more and more about my Rover. Thanks for the info.

 

Sorry, my '97 has four so thats what I was going by.

 

Hahahahah. So I'm confused again. Do I have 2 or 4 o2 sensors in my 94 Disco?

 

There is only one way to know for sure, go and look.

 

LOL. Thats exactly what I'm gonna do after work today.

 

When you are looking.... If your truck has 2 (two) they will be in the exhaust pipes between the exhaust manifolds and the catalytic converters. If it has 4 (four), 2 of them will be before catalytic converters, and the other 2 will be after the catalytic converters...so If you do not see any after the catalytic converters (between catalytic converters and muffler) then you only have 2.

 

Judging by Atlantic British's inventory, it looks like the 94-95s use two o2 sensors while the later years use four sensors. Sigh, something else I should replace. Speaking of, I have a pass. side mirror and some hoses to replace.

 

Looking at the link and looking at my truck I have the gems engine (or at least the intake is the same) and four 02 sensors. I have a D1 (96 4.0)