I purchased a volvo ec210lc. It's my first volvo machine. It is throwing errors for boom up, boom down when I try to use it. I Googled how to get I to service mode and discovered indications over 600psi with the machine shut down. I located the sensors and noticed that several were black plastic but 2 of them were metal with a volvo tag. I thought I had an issue caused by junk knock off sensors so I ordered a $300 sensor from the dealer and plugged it into the boom up wire. It gave me a reading of 638psi. Now mind you I didn't screw it into the hydraulic line, I just plugged it into the wires!
What are the errors you were chasing down in the first place? In service mode, scroll down and click on the message tab.
As @funwithfuel requested please post the error codes that you have. I suspect you have an issue with the ground circuit for those 4 sensors as they share power and ground from the VECU. Probably you will see MID187 PPID1423,24,29,30 FMI3 when you check error codes.