I think my friend screwed...
 

I am turning to you Volvo Experts for some help.

I have a 2000 S80 T6 that i am working on for a friend. He bought the car knowing it had problems but i don't think he realized how bad it is.

Right now it will start and idle smooth and it is "Driveable". It does not shift correctly. The Tracs Disconnected light is on and the engine light.

The "P" codes with the Volvo code that it has are: (definitions are from ALLDATA)

P0118/ECM-2000- I have replaced the coolant temp sensor with a new one. Code still active and coolant temp gauge does not read.

P0113/ECM-1200- I have not changed the IAT sensor

P0154/ECM-290A-I have not changed the O2 Sensor. (is there and easy way to change the Upstream sensor with out removing the exhaust?)

P0103/ECM-1300-I have put a brand new MASS Air Flow on the car.

P0236/ECM-170F- I have no idea what this is???

P0238/ECM-1600-I think this is the TCV and have not changed it

P0452/ECM-4020-Fuel tank pressure sensor and i have not done anything for this.

While driving the car at random times the engine RPM will fluctuate randomly and after the care gets warm the low oil pressure light comes on.

What i have done so far is replace all Plugs and two new coils, New Coolant temp Sensor, Mass Air Flow, Oil and filter, Transmission Fluid and filter.

The guy he bought the car from has had the timing belt replaced as well as the cam sensor. I checked and the timing marks and they seem to be spot on.

If you have any ideas of what I should do next that would be great.