SPN 173 FMI 6: Meaning and Fix
SPN 173 FMI 6 indicates excessive current or grounded circuit in the exhaust temperature sensor monitoring system. This fault commonly appears during aftertreatment system diagnostics when technicians are troubleshooting DPF regeneration failures or following ECM replacement procedures. The engine control module detects abnormal electrical characteristics that exceed predetermined threshold values, potentially compromising exhaust temperature monitoring accuracy and triggering protective engine responses.
Common Symptoms
- Engine Derate: Reduced power output due to ECM protective response from unreliable exhaust temperature data.
- DTC Storage: Multiple related fault codes stored in ECM memory affecting aftertreatment system operation.
- DPF Issues: Failed regeneration cycles caused by inability to accurately monitor exhaust gas temperatures.
- Warning Lights: Check engine lamp illuminated with possible DEF system warnings on dashboard display.
Probable Causes
- Damaged Harness: Corroded or melted wiring creating short circuit conditions in sensor signal pathway.
- Faulty Sensor: Internal thermocouple failure causing electrical resistance changes and current flow abnormalities.
- ECM Malfunction: Engine control module input circuit damage from voltage spikes or water intrusion.
- Ground Fault: Sensor wire touching chassis ground creating unintended current path through exhaust components.
Advanced Technical Analysis
The ECM continuously monitors exhaust temperature sensor current draw through dedicated analog-to-digital converter channels. When measured amperage exceeds 250 milliamps for longer than 2.5 seconds, the microcontroller flags FMI 6. This threshold protects both sensor circuitry and ECM input stages from thermal damage caused by excessive current flow.
Grounded circuit conditions create alternative current pathways that bypass normal sensor resistance values. The ECM’s internal pull-up resistors detect these anomalies through voltage divider network analysis. Debouncing algorithms require sustained fault conditions before code activation, typically 15-20 engine cycles, preventing false triggers from momentary electrical interference or vibration-induced contact issues.
Upon fault confirmation, the ECM implements graduated torque reduction strategies beginning with 10% power limitation. If exhaust temperature monitoring remains compromised beyond 50 engine hours, maximum derate reaches 40% to prevent thermal damage. Substitute temperature values derived from injection timing and load calculations maintain basic engine protection during sensor failure conditions.
Workshop experience demonstrates this code frequently appears after pressure washing engine compartments or following exhaust system repairs. Technicians report success using oscilloscope analysis to identify intermittent grounding issues that multimeter testing misses. Preventive maintenance includes annual harness inspection and dielectric grease application at connector interfaces, particularly on construction equipment operating in corrosive environments.
Step-by-Step Troubleshooting Guide
- Circuit Inspection: Visually examine exhaust temperature sensor harness for damage, corrosion, or contact with hot surfaces.
- Resistance Testing: Measure sensor resistance and continuity using digital multimeter with engine temperature compensated values.
- Ground Verification: Check for unwanted ground paths between sensor signal wire and chassis using insulation tester.
- ECM Validation: Substitute known good sensor and monitor live data to confirm ECM input circuit functionality.