SPN 5491 FMI 6: Meaning and Fix
SPN 5491 FMI 6 indicates the ECM detected excessive current on the Aftertreatment 1 DEF Line Heater Relay 1 output, signaling a short-to-ground or failed relay coil. This code commonly appears after a forced DPF regeneration in subzero temperatures when the heater relay remains energized too long, melting the wiring harness insulation against the frame rail. Technicians frequently encounter this fault after replacing the ECM without verifying relay coil resistance.
Common Symptoms
- DEF Line Icing: DEF line remains frozen at startup, preventing dosing and triggering SCR derate.
- Engine Derate: ECM reduces torque by 25% to protect aftertreatment from incomplete DEF injection.
- Dash Warning Lamp: Amber warning lamp illuminates with a DEF system fault message on display.
- No DEF Dosing: SCR system disables dosing to avoid overpressure or pump damage.
Probable Causes
- Relay Coil Short: Internal winding short in relay 1 draws excessive current, triggering FMI 6.
- Harness Chafing: Insulation wear on positive wire near exhaust bracket causes ground contact.
- Corroded Connector: Moisture ingress at relay socket pins increases leakage current over threshold.
- ECM Driver Failure: Faulty high-side driver in ECM outputs sustained overcurrent without shutoff.
Advanced Technical Analysis
The ECM monitors the relay control pin voltage via a sense resistor. If the current exceeds 2.5 A for more than 500 ms, the diagnostic sets FMI 6. The microcontroller then latches the relay off to prevent thermal damage to the PCB traces. This logic is based on SAE J1939-71 and Bosch ED17+ strategies.
A short-to-ground condition often occurs when the relay output wire (typically 18 AWG) abrades against the DEF line heater bracket. Vibration and salt corrosion reduce insulation resistance below 10 kΩ, causing a current spike that the ECM’s debouncing timer registers as a hard fault.
Upon detecting FMI 6, the ECM de-energizes the relay and sets the Aftertreatment 1 SCR Dosing System to a safe state. Torque is derated to 40% of demand, and DEF dosing is inhibited. The fault must be cleared via scan tool after repair, as per MAN and Deutz service protocols.
Long-term prevention includes routing the harness away from exhaust components and applying dielectric grease to the relay connector. In workshop practice, this code frequently follows a melted harness caused by an aftermarket block heater. Always measure relay coil resistance (30-60 Ω) before replacing the ECM.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check relay socket and harness for melted insulation, chafing, or corrosion near exhaust.
- Resistance Measurement: Measure relay coil resistance between pins 85 and 86; should be 30-60 Ω.
- Voltage Drop Test: Key-on, engine-off: measure voltage at relay output pin 30 to ground; expect <0.5 V.
- ECM Driver Test: Disconnect relay and command on; verify 12 V at ECM pin with a test light.