What does the error code PO300B refer to in a volvo 1996 850 GLT?

code p 0440 refers to evaporative emission (EVAP) system---malfunction===causes ---hose connections ---intake leak---EVAP canister purge valve
probably the purge valve needs replacing

code p 0172 refers to system too lean, bank 1===causes --intake blocked , EVAP canister purge valve ---fuel pressure ---EGR system---injector/s HO2S
you will notice that the purge valve is in both codes and it also affects the fuel pressure by not venting the tank correctly
That will account for the fuel pressure problem
I would seek advice from an accredited service specialist that works on your type of car but I would be paying attention to the common fault ( purge valve) before any thing else

15 ft lbs then angle torque 90 degrees

code p0102 refers to MAF/VAF sensor ---low input ===causes --wiring short to ground. MAF/VAF sensor
code po172 refers to system too rich --bank 1=== causes ---intake blocked --EVAP canister purge valve---fuel pressure ---HO2S sensor
code po random misfires ---plugs --wiring ---injectors--low compression
code po305 refers to cylinder 5 misfire detected ===causes --engine mechanical fault--ignition/ fuel system ---injector ---ECT/MAF

I would check that the air filter is not blocked first as it affects the MAF/VAF sensors . Have a fuel pressure system pressure test done

You will need to turn the ignition to the number 1 position and turn the radio on. It can take several hours, but the word code should come back on. When that happens, you will need to put the correct code in. If you don't have it, a Volvo dealer can give you the correct code from your VIN.

Use the shop manual locate its position on the camshaft check it for wear and voltage and any other checks the shop manual says to do.

I just got the security code for mine. 6626...you can try it. Or go down to your local Volvo dealer with your registration. They can look up the code using your VIN number.

go to your local volvo dealership, and you must have the car in your name, but ask one of the mechanics if you can have the code for your radio, they'll look it up and give it to you I believe.

VOLVO STARTED USING THIS SYSTEM IN 1996 AND 1997 850 MODELS IT IS AN AIR PUMP --------------SMOG PUMP TO MEET EPA EMISSIONS THE DIAGRAM OF THE SYSTEM IS LOCATED UNDER THE HOOD AND SHOULD TELL YOU WHERE THE VALVE IS LOCATED THERE ARE MANY ECU SYSTEMS USED ON VOLVOS AND THIS SHOULD PUT YOU IN THE RIGHT DIRECTION IF YOU STILL HAVE A PROBLEM GIVE ME THE 2 BIG NUMBERS ON THE ENGINE MANAGEMENT ECU

If it's the volvo fitted sound system contact any volvo main dealer with proof of I.D they have codes on file

If you need the security code, you need to call a Volvo dealer and give them the VIN number, provided it is the same stereo the car came with.  Most dealers will do it for free, if you can't find one, call Boston Volvo Village.