SPN 676 FMI 6: Meaning and Fix
SPN 676 FMI 6 indicates the Engine Glow Plug Relay circuit has detected current above normal or a grounded condition. This fault commonly appears after a failed cold-start attempt in sub-zero temperatures, when the relay remains energized longer than intended due to a shorted glow plug or chafed harness. It can also surface after an ECM replacement if the relay driver circuit is not properly recalibrated per OEM specifications.
Common Symptoms
- Hard Cold Start: Engine cranks but fails to ignite below 0°C due to inactive or shorted glow plug relay.
- White Exhaust Smoke: Excessive unburned fuel vapor from cold cylinder walls, visible as dense white smoke at startup.
- ECM Warning Lamp: Red or amber MIL illuminated on dash, often accompanied by a logged active or inactive fault code.
- Battery Drain: Parasitic load on batteries when relay remains stuck closed, causing rapid voltage drop overnight.
Probable Causes
- Shorted Glow Plug: Internal short in one or more glow plugs draws excessive current, tripping the relay driver.
- Harness Chafing: Insulation wear on the relay control wire causes direct contact with engine ground.
- Relay Coil Failure: Shorted winding inside the relay itself results in continuous current flow beyond the ECM limit.
- ECM Driver Damage: Overcurrent event has permanently damaged the internal MOSFET or driver circuit in the ECM.
Advanced Technical Analysis
The ECM monitors the glow plug relay control circuit using a high-side driver with integrated current sensing. When the commanded duty cycle produces a feedback current exceeding the calibrated threshold (typically 10-15A for a standard 4-cylinder set), the microcontroller sets SPN 676 FMI 6 within 50 ms to protect the driver.
A debouncing timer of 200 ms prevents false triggers from transient inrush currents. If the overcurrent condition persists beyond this window, the ECM latches the fault and de-energizes the relay. This logic ensures that a single intermittent short does not permanently disable the glow system.
Once the fault is active, the ECM disables the glow plug relay output entirely and sets a torque derate of 25% to prevent unburned fuel from entering the exhaust. The aftertreatment system may also inhibit regeneration until the fault is resolved, as seen in MAN D26 common rail applications.
Long-term diagnostics require verifying glow plug resistance (0.5-2.0 ohms cold) and insulation to ground (>10 MΩ). Technicians frequently encounter this code after a forced DPF regeneration when the relay overheats and the coil winding shorts. Replacing all glow plugs as a set prevents recurrence, as seen in Deutz TCD 2013 workshop bulletins.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check relay harness for chafing, burns, or corrosion near the valve cover and engine block.
- Resistance Test: Measure each glow plug resistance between terminal and ground; any below 0.5 ohms indicates a short.
- Relay Bypass Test: Energize relay directly with 12V; if current exceeds 20A, replace the relay assembly.
- ECM Output Check: With relay disconnected, measure ECM control pin voltage; should pulse 12V during pre-glow cycle.