Massey Ferguson U 1388 – SCR (Selective Catalytic Reduction) / DEF system controller: Urea pressure sensor issue

Fault code U 1388 indicates an issue with the Urea pressure sensor (part of the Diesel Exhaust Fluid / AdBlue system). It can log as a shortcut to Battery+, a shortcut to Ground, or supply voltage above upper or below lower limit.

• Massey Ferguson 8600 Series
• MF 8670
• MF 8690
• Massey Ferguson 6700 S Series

1. 1

   Check the urea fluid sensor power supply circuit using a scan tool and multimeter. Test the wiring harness for a short circuit to Ground or a short circuit to Battery+.

2. 2

   Verify that the supply voltage to the sensor is within the specified upper and lower limits.

3. 3

   Inspect the DEF/AdBlue tank, pump/head module, and heated lines (supply and return). Look for kinks, rub points, or damaged O-rings that could cause air ingress or pressure faults.

4. 4

   Ensure the DEF tank contains a clean, precise 32.5% urea solution. Contamination (dirt, diesel, or water) can trigger secondary pressure and quality codes.

1. 1

   Isolate the Circuit

   Disconnect the battery/power supply before servicing the SCR system electrical components.

2. 2

   Repair Wiring

   If the diagnostic checks reveal a short to ground or battery in the console harness, repair or replace the affected section of the urea fluid sensor power supply circuit.

3. 3

   Replace the Sensor

   If the wiring harness and supply voltage are intact but the U 1388 code remains active (indicating an internal sensor failure), replace the Urea Pressure Sensor / DEF Sensor.

4. 4

   System Purge and Prime

   After replacing the sensor or repairing lines, ensure the system is properly sealed with new O-rings to prevent air leaks.

5. 5

   Clear Codes

   Reconnect power, turn on the ignition, and use a diagnostic scan tool to clear the U 1388 fault code and verify that the sensor is reading the correct pressure.