The Hyster forklift fault code (often paired with SPN 3701 and FMI 15) indicates that the Diesel Particulate Filter (DPF) has reached Level 3 soot accumulation .
The sensor that measures the pressure difference (the Delta-P sensor) could be faulty. If this sensor fails, it might send a signal to the ECU claiming the pressure is extremely high, even when the filter is clean. Before replacing an expensive filter, always test the sensor. hyster forklift fault code 3701-15