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