Any air coming into the system after the MAF/AMM will cause a rich condition at the spark plugs. Initially, the computer sees a lean burn at the front O2 sensor and adjusts the fuel to increase to make up for it. Often associated with a long term lean/rich error, which may or may not trigger a fault.

The rear O2 sensor is just a monitor to make sure the front O2 sensor is doing its job. Some scanners can follow the O2 sensors and graph the electrical flow, but a P0140 is a wiring or plug problem. If the mechanic didn't unplug the sensors before removing them, the wires will get twisted badly and stretched. If the plug end was removed, there may be dirt or grease in the area causing a bad contact.

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My back feels better when I sit in a Volvo seat

It looks like it shares a fuse with other system(s):

http://www.matthewsvolvosite.com/forums/viewtopic.php?f=1&t=50864

So if it WAS a blown fuse, you would probably be lacking other systems also.

Curious - do you still have the original oxygen sensors laying around? If so, can you read the brand name? Some of these cars,generally the non-turbos I think, use Denso or maybe even NTK oxygen sensors. There have been many web posts over the years that suggest if you don't replace them with an exact plug-and-play OE part that you are flirting with incompatibility/failure - that means no "universal fit" or "adapted" parts. I just can't find anywhere on the web that spells out who the OE manufacturer is for your particular sensors.

Also, did you replace both sensors at the same time? Any chance you have them mixed up?
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (fixed the ignition - now back to the brakes again)