SPN 4077 FMI 5: Meaning and Fix
SPN 4077 FMI 5 indicates the ECM detected current below normal or an open circuit on the aftertreatment 1 fuel pressure sensor 2 input. This code commonly appears after a forced DPF regeneration when a technician accidentally damages the sensor harness. The sensor measures secondary fuel pressure for burner-based regeneration systems. An open circuit prevents accurate pressure feedback, disabling active regeneration and potentially leading to diesel particulate filter clogging.
Common Symptoms
- No regeneration: Aftertreatment system cannot initiate active regeneration due to missing pressure signal.
- DTC lamp ON: MIL or amber warning lamp illuminates immediately after key-on or during self-test.
- Engine derate: ECM reduces torque by up to 25% to limit soot production and protect the DPF.
- Fault logged: The code is stored as an active or inactive fault in the ECM memory.
Probable Causes
- Open sensor wire: Broken or corroded signal wire between sensor and ECM connector pin.
- Sensor failure: Internal open circuit in the pressure sensor due to thermal stress or contamination.
- ECM connector issue: Loose or corroded terminal at ECM 120-pin connector causing intermittent open.
- Harness chafing: Worn insulation on harness near exhaust manifold causing signal wire break.
Advanced Technical Analysis
The ECM monitors the sensor signal line for current flow using a pull-up resistor to 5V. When the circuit is open, the voltage at the ADC input rises to 5V and current drops below the 4-20 mA threshold. The ECM sets FMI 5 after a debounce timer of 2.5 seconds to filter transient noise. This logic follows SAE J1939-73 diagnostic message 1 (DM1) protocol.
Electrical analysis reveals that an open circuit typically shows >4.9V on the signal line when measured with a DMM. The ECM’s internal diagnostic compares the raw ADC value against a calibrated open-circuit threshold. In Deutz TCD 2.9 engines, the aftertreatment fuel pressure sensor 2 operates at 0.5-4.5V linear output; an open circuit forces the voltage to the 5V rail, triggering the fault.
When FMI 5 is active, the ECM disables the aftertreatment fuel dosing valve and enters a safety fallback. Torque is derated by 15-25% depending on OEM calibration (e.g., MAN D26 common rail). The regeneration inhibit flag is set, preventing burner operation. This protects the DPF from thermal damage but requires immediate repair to avoid soot overload.
Long-term prevention includes inspecting the harness routing near the exhaust manifold during every DPF service. Technicians frequently encounter this fault after replacing the ECM without transferring the sensor ground wire correctly. A proactive step is to apply dielectric grease to all aftertreatment sensor connectors. In Bosch ED17 systems, a pin drag test on the ECM connector is recommended every 500 hours.
Step-by-Step Troubleshooting Guide
- Visual inspection: Check sensor connector and harness for cuts, burns, or corrosion near the exhaust.
- Voltage measurement: Measure signal pin voltage at sensor: should be 0.5-4.5V; >4.9V indicates open.
- Continuity test: Disconnect ECM and sensor; check resistance of signal wire (< 5 ohms indicates good).
- Sensor substitution: Replace sensor if wiring passes tests; clear fault and verify regeneration operation.