SPN 111 FMI 6: Meaning and Fix
SPN 111 FMI 6 indicates the ECM detected current above normal or a grounded circuit in the engine coolant level sensor. This often occurs after a coolant change or expansion tank replacement when the sensor connector is accidentally pinched or exposed to conductive coolant residue. Technicians frequently see this fault after an engine wash or coolant top-up using non-distilled water.
Common Symptoms
- Erratic gauge reading: Coolant level gauge shows full or empty intermittently due to shorted sensor signal.
- Constant low-level alarm: Dash displays persistent low coolant warning even when expansion tank is full.
- Engine derate active: ECM reduces torque by up to 25% to protect engine from potential overheating.
- No start condition: Some OEM calibrations inhibit engine cranking when coolant level sensor circuit is faulted.
Probable Causes
- Shorted sensor wiring: Insulation chafed against engine block or frame causing signal wire to ground.
- Corroded connector pins: Coolant wicking into connector creates conductive path between pins and ground.
- Failed internal sensor: Level probe internal electronics shorted due to thermal stress or vibration fatigue.
- ECM pin damage: Corrosion or bent pin at ECM harness connector causes ground fault detection.
Advanced Technical Analysis
The ECM applies a 5V pull-up resistor to the coolant level sensor signal line. Under normal conditions, the sensor pulls the line to a voltage between 0.5V and 4.5V depending on coolant level. When FMI 6 triggers, the ECM detects the signal line voltage dropping below 0.25V for longer than the debounce timer (typically 200ms), indicating a short to ground. The ECM averages three consecutive samples before logging the fault.
Electrical analysis reveals that a grounded circuit creates a current path exceeding 10mA through the pull-up resistor. The ECM’s analog-to-digital converter reads the voltage as zero, bypassing the normal level-to-resistance curve. German OEM documentation (MAN TGL/TGM) specifies that the fault must persist for two consecutive drive cycles before the ECM activates the MIL. A single intermittent short may clear after ignition cycle.
Upon detecting FMI 6, the ECM enters a fail-safe mode: it sets the coolant level to 0% and illuminates the stop lamp. Torque is derated linearly to 40% over 60 seconds if vehicle speed exceeds 5 km/h. The ECM also disables the engine protection shutdown override, forcing the operator to stop. Bosch ED17 ECUs further log a freeze frame of coolant temperature and engine speed at fault occurrence.
Long-term prevention requires inspecting the sensor harness routing near the radiator fan shroud, where chafing is common. Technicians should measure resistance between sensor signal pin and chassis ground with the connector disconnected; a reading below 10kΩ indicates a short. In Deutz TCD 2013 engines, replacing the sensor gasket and dielectric greasing the connector reduces recurrence. Always verify the expansion tank part number matches OEM spec after replacement.
Step-by-Step Troubleshooting Guide
- Visual inspection: Check sensor connector and harness for chafing, coolant contamination, or corrosion near engine block.
- Resistance measurement: Disconnect sensor; measure signal pin to ground. Below 10kΩ confirms short; above 100kΩ is normal.
- Voltage check at ECM: Backprobe ECM pin; ignition on. Voltage below 0.5V indicates grounded circuit; 5V indicates open.
- Connector replacement: Replace sensor pigtail if corrosion found. Use Deutsch DT series connectors with integrated seal.