What I suggest before you spend lots of money is to obtain a vacuum elbow replacement kit and replace all the rubber vacuum elbows. I had the same particular problem and the solution was the one vacuum elbow under the intake manifold next to the water thermostat on the left (passenger) side of the car. Very difficult to get to. Also, there is a very long rubber hose also under the intake manifold that if you remove it to replace the rubber elbow, I strongly suggest you replace this hose. It gets brittle and cracks and can lead to intermittent problems.

I got so frustrated that I replaced all the rubber vacuum hoses and elbows.

One other thing I did, since I also kept getting trouble codes associated with exhaust, intake, vacuum, and fuel injectors (after replacing each one), which actually fixed most of my other problems, was I disconnected the battery, and disconnected all the electrical connectors (and I mean each one) in the engine bay. They were all severely corroded. I used electronic contact cleaner and cleaned and burnished each connector's contacts.

Guess what? that was what was causing the intermittent problems with my fuel injectors causing a valve error code.

All new rubber hoses and elbows and clean and burnish all connectors and the car runs excellent now.

If you're running rich, perhaps a good O2 sensor will set that code?
You're right to diagnose carefully before throwing expensive parts.

Here's some ideas after you check compression.
excerpts from "FuelControl850-70series.pdf"
"There's a direct relationship between the correct fuel quantity delivered to the engine
by the injectors and the fuel pressure." So you need to verify fuel pressure.
Triple check for air leaks after the MAF. I got hoodwinked there before with a very subtle tear in the bellows.
and another time the clamp wasn't set right, don't ask how that happened.

Key Sensors: RPM, MAF IAT (intake air pressure) VAT Volume Air Flow Sensor (Turbo)
some turbo engines have an Exhaust Temp Sensor that controls full load enrichment.
These can all get dirty or drift off proper operating specs with age besides outright failure
That might lead to running rich and or low gas mileage, and be hard to diagnose.
There's a Volvo service page that explains component checking,
"SystemComponentsTest.pdf" Pg 7 is "Fuel Pressure Regulator test"

How long can injectors go without service, and how do you diagnose? There's a resistance range
that they give but that must just indicate an open injector solenoid winding I'm guessing, total failure.

Just some ideas, focusing on diagnosis of the root problem.

Bill

When I get this on my 850 turbo, it has usually meant time to do the O2 sensors, so I agree with you on that diagnosis.

However, I tend to reset (as you did) and wait for reoccurrence.

Your reoccurrence seems to have come a lot sooner than mine did. I think it would go maybe a week or two before it would reoccur. (It takes a while for the ECU to decide that an O2 sensor is lazy, they drift slowly.)

Since yours seems to have come on suddenly, and reoccurs quickly, that makes me think vacuum leak. Something broke or split. I'd look for something like that first, then replace the O2 sensors if a second, close look didn't reveal any leaks.

As you know, they age together. You could replace just one, but the other will follow soon after, so I usually do both together just to minimize the hassle.

Hope this helps!

Roger