I am geeting error code 48 on my phaser 7750dn

That is a fan failure code, Either the Fuser fan ( most likely) behind the back cover near the front.Low voltage power supply fan or rear cover fan is not operating.
Most of the time it is the fuser fan that is not running or even if its running slow it will give you that error code.

Here are the notes
Problem:

Fan Failure Message (Code / Error 48)

The machine logic detected a failure of the Fuser fan, LVPS fan or rear fan.

1. Insure there is adequate ventilation around printer. Refer to Section 1 of Service Manual for required clearances.
2. Follow the procedure in Chapter 3 for this error.
3. This problem can be very intermittent. Fans can work in diagnostics and fail in the middle of large print jobs. If unable to duplicate problem replace all 3 fans.
4. Use service diagnostics to verify the Fuser, LVPS, and ROS fans. These fans have a third wire which provides feedback to the Engine Board.
5. Visually check each or these fans for a build up of dust or toner.
6. There is a correction to this procedure in the Service Manual. The fans that can cause this error are the Fuser, LVPS, and ROS(laser fan). Not the Rear fan. The rear and bottom (developer) fans are not monitored by the system and won't generate this error.

• FAN ASSEMBLY, FUSER (63%)
• FAN ASSY; LOW VOLTAGE POWER SUPPLY (45%)
• FAN REAR ASSEMBLY (Laser Fan) (25%)