SPN 5442 FMI 6: Meaning and Fix
This fault indicates the ECM detected a current exceeding the normal range on the fuel injection pressure sensor circuit for multiple cylinders, suggesting a short-to-ground or short-to-power condition. In practice, this code often appears after a wiring harness chafes against the engine block during a recent turbocharger replacement, causing an intermittent ground fault that sets the code under load.
Common Symptoms
- Engine Misfire: Multiple cylinders misfire due to erratic fuel injection pressure signals, causing rough idle and hesitation under load.
- Power Loss: Significant torque reduction as ECM limits fuel delivery to protect injectors from overpressure or underpressure events.
- Hard Starting: Extended cranking time because the ECM cannot verify correct rail pressure, preventing injection enable.
- Diagnostic Lamp On: Red or amber warning lamp illuminates immediately; fault logged with active current monitoring counter.
Probable Causes
- Short-to-Ground: Insulation failure on sensor signal wire contacting engine ground, often at cylinder head or fuel rail brackets.
- Faulty Sensor: Internal short in pressure sensor due to diaphragm rupture or electronic component failure from vibration.
- ECM Damage: Internal driver failure in ECM output stage, causing continuous current draw above 100 mA.
- Harness Chafing: Wiring harness rubbed through against sharp edges of intake manifold or exhaust manifold heat shield.
Advanced Technical Analysis
The ECM monitors fuel injection pressure via a 5V reference sensor with a 0.5-4.5V analog output. When current exceeds 20 mA (normal 5 mA), FMI 6 triggers. The microcontroller compares instantaneous current against a 16-sample moving average; a deviation above 15 mA for 200 ms confirms the fault. This logic prevents false triggers from transient noise during injector actuation.
Electrical analysis reveals a debouncing timer of 500 ms before fault activation. If the current spike lasts less than 500 ms, the fault is stored as an inactive code. Real-world cases show that a chafed wire touching the engine block during cold start (higher vibration) can cause intermittent spikes that eventually become active after the timer threshold is exceeded.
Upon activation, the ECM enters a safety fallback: it commands a 40% torque derate and sets injection timing to a fixed map. Fuel rail pressure is limited to 800 bar (instead of 1800 bar) to prevent overpressure injury. The ECM also disables cylinder cut-out tests to avoid misdiagnosis. This protects injectors but forces immediate repair to restore full power.
Long-term prevention includes inspecting the harness routing per Deutz factory manual TPI 2019-004, which specifies zip-tie spacing of 150 mm near the fuel rail. Technicians frequently encounter this fault after replacing the ECM without checking the sensor harness; a simple continuity test to ground with a 12V test light (non-LED) can identify the short before ECM replacement.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check harness from ECM connector C1 pin 23 to sensor for chafing, burns, or moisture ingress.
- Resistance Test: Measure resistance between sensor signal wire and engine ground; should be >1 MΩ. Below 10 Ω indicates short.
- Sensor Swap: Replace fuel injection pressure sensor with known-good unit; clear fault and test under load.
- ECM Test: Disconnect sensor; measure current draw from ECM signal pin. If >20 mA, replace ECM per Bosch repair procedure.