A lumber yard reported that a Toyota 8FDU25 diesel forklift (4D112 engine) suddenly lost power, top speed was limited to 5 km/h, and the dashboard flashed a "DPF Regeneration Required" warning. The operator initiated a parked regen, but it aborted after 5 minutes, and the engine derated further.
The technician connected the Toyota diagnostic laptop and found a "DPF Differential Pressure Sensor Circuit High" code. He checked the live data: the differential pressure across the DPF (Diesel Particulate Filter) read 15 kPa, even at idle, indicating a severely blocked filter. However, he removed the DPF pressure tubes and found them dry and clear.
To verify the blockage, he installed a temporary manual manometer across the DPF. The actual physical pressure was only 2 kPa at idle. The electrical sensor was reading falsely high. Upon removing the sensor, he found the electrical connector packed with moisture and green corrosion from pressure washing. The corrosion had shorted the 5V reference pin to the signal pin, sending a constant 4.8V signal to the ECM. The ECM falsely calculated that the DPF was 100% full of soot and severely derated the engine fuel map to force the operator to stop. When the auto-regen was initiated, the ECM detected the "full" status but could not raise the exhaust temperature sufficiently because the engine was heavily derated, causing the regen to abort.
The repair involved replacing the shorted differential pressure sensor, cleaning the wiring harness connector, and applying dielectric grease. The DPF was physically clean and did not require baking. After clearing the fault codes and resetting the soot accumulator in the ECM, the forklift regained full power and top speed, and the regen warning cleared.
