Doosan Stage V Machine U1028 – DEF Quality Sensor Open Circuit Solution

This guide explains how to diagnose and repair the Doosan Stage V machine fault code U1028, which indicates a DEF (Diesel Exhaust Fluid) quality sensor open circuit.

Required Tools and Preparations

Before starting diagnostics, ensure the following tools are available:

• Doosan uVIM Diagnostic Tool Excavator Forklift Engine Fault Diagnostics with DMS-5 Software V2025.08
• 2025.08 DOOSAN (DEVELON) DMS 5 -3.2.8 Diagnostic Software
• Doosan Excavator Inspection Diagnostic Tool DDT SCR DPF G2 Scan DCU ECU DMS-5 Hardware + Software V2025.08

1. Fault Overview

• Fault Code: U1028
• SPN/FMI: E00352003
• Fault Type: Electrical fault – Open circuit

Possible Reasons

1. Defective DEF quality sensor
2. Wiring harness fault inside the DEF module
3. CAN wiring harness insulation damage
4. Incorrect resistance in CAN circuit

System Effect

• Inducement Group: Group 5 (Tampering)
• Emissions control strategies may be activated

Note: Pin assignments vary depending on the DEF tank type used on the machine.

2. Component Location

• The DEF tank, DEF quality sensor, and electrical connectors vary by machine model and application
• Refer to the machine-specific service manual to identify the exact sensor and connector location

3. Conditions for Running the Diagnostic

• The diagnostic runs continuously at every sampling interval
• No CAN bus-off condition
• No communication timeout errors present

4. Conditions for Setting the Fault Code

• Fault code U1028 is set when the FMI equals 3, indicating an open circuit in the DEF quality sensor system

5. Conditions for Clearing the Fault Code

• The fault is automatically cleared when the FMI is no longer equal to 3
• Normal circuit operation must be restored

6. Diagnostic Checklist

Follow these steps to identify and repair the issue:

1. Inspect the DEF quality sensor for physical damage
2. Check the DEF module wiring harness for open circuits or broken wires
3. Inspect CAN wiring insulation for damage or corrosion
4. Measure circuit resistance and compare with Doosan specifications
5. Verify connector pin integrity and proper seating
6. Replace the DEF quality sensor if faulty
7. Clear the fault code and confirm normal operation