The FI unit is always a Bosch unit. Either a LH 2.2 or a LH 2.4 system. The igntion system can differ. If you have a Bosch ignition there should be a round coil on the RH strut. If you have a Regina there should be a ignition unit with built-in coil on the LH strut. It looks like a transformer. With Bosch ignition there is a separate ECU above your accelerator pedal. Remove the footwell prtoection first.

JohnB is right about the location, just above and to the right of the passenger's right foot. Access is a little more involved than pulling back the carpet, but very doable. I don't see any need to get to the ECU if what you need is an O2 sensor, however.

If your 740 is a non-tubo sold anywhere in North America except California, than it has Rex-Regina engine management. Info on how to tell, and how to get to the ECU, are detailed in the 700/900 FAQ under LINKS at the top right of this page. Read all about it.

Check out this recent thread for what O2 sensor to get for Regina and where you can get one less expensively than I have ever seen elswhere (I just ordered, but haven't received yet).

http://brickboard.com/RWD/index.htm?id=1040934

Do NOT believe the sites that tell you that the generic, splice in, O2 sensor is OK for Regina. Those are zirconia sensors, and fine for the Bosch 740s, but incompatible with Regina.
--
Andy in St. Paul - '91 745 213K mi, '91 745 210K, '90 744 183K, all Rex-Regina - past 240s

Look at the ignition coil. If it doesn't look like the cylinder you're used to, and has a finned heat sink, it's a REX (ignition), REGINA (FI).

For Regina O2 sensor, look for an NTK 25002. Near as I can tell, it's the correct (titania-based) one. Cheapest I've seen is $99.85 at sparkplugs.com.
--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

All the computers on the 90 740XX are on the RHS of the passenger footwell. Peel back the carpeting.