John Deere 004334.03 – ECU: DEF System Pressure Sensor Voltage High

The Engine Control Unit (ECU) has detected a voltage signal from the DEF system pressure sensor that is higher than the normal operating range. This indicates a potential issue with the sensor itself, its associated circuitry, or the DEF pump assembly. The sensor monitors the pressure of the DEF being supplied to the dosing valve, and abnormally high voltage readings typically point to a sensor fault or a short in the wiring.

• 5085E
• 5085M
• 7200R
• 7215R
• 7230R
• 8225R
• 8235R
• 8245R
• 8260R
• 8270R
• 8285R
• 8295R
• 8310R
• 8320R
• 8320RT
• 8335R
• 8345R
• 8345RT
• 8360R

1. 1

   Check for multiple diagnostic codes using a scanner to see if other DEF system or sensor circuit codes are present

2. 2

   Inspect DEF quality visually (should be clear with a faint ammonia smell) and test concentration with a DEF refractometer (should be 31.8% to 33.2% urea)

3. 3

   Inspect the wiring harness and electrical connectors for the DEF pressure sensor and DEF pump/dosing unit for damage, corrosion, bent pins, or loose connections

4. 4

   Check the DEF system lines (especially the suction line) and suction manifold filter for leaks, crystallization, or blockages

5. 5

   Perform a voltage test across the sensor's power, ground, and signal wires to check for a short to a power source in the signal wire

1. 1

   Address preliminary findings

   Repair any damaged wires or connectors. Clean any blockages and replace filters as needed. If DEF is contaminated, drain the tank, flush with distilled water, and refill with fresh fluid. Clear codes and retest.

2. 2

   Isolate the sensor

   Disconnect the DEF pressure sensor's electrical connector to prepare for circuit testing.

3. 3

   Check for fault code change

   With the sensor disconnected, turn the ignition on (without starting the engine). If the FMI 3 (Voltage High) code changes to an FMI 5 (Current Below Normal/Open Circuit), it strongly indicates a faulty pressure sensor.

4. 4

   Replace the DEF pressure sensor

   If the sensor is determined to be faulty, replace it. Note that in many systems, the sensor is integrated into the DEF pump or dosing unit assembly, which may necessitate replacing the entire unit.

5. 5

   Clear codes and verify repair

   Clear all diagnostic trouble codes using a diagnostic scanner. Start the engine and perform a full regeneration cycle of the aftertreatment system to validate the repair.