SPN 173 FMI 4: Meaning and Fix
This fault indicates the engine exhaust temperature sensor circuit has a voltage below normal or is shorted to ground. The sensor, typically a 2-wire NTC thermistor, reports temperatures from -40 to +850 °C. In practice, this code often appears after a forced DPF regeneration when the sensor harness is melted against the exhaust pipe, or after an ECM replacement where the sensor ground wire is pinched.
Common Symptoms
- Engine Derate: ECM reduces torque by up to 40% to protect aftertreatment components from unmonitored exhaust heat.
- DPF Regen Inhibited: Active regeneration is blocked because the ECM cannot verify exhaust temperature rise above 250 °C.
- Check Engine Lamp: Red stop lamp or amber warning lamp illuminates immediately upon key-on, depending on OEM configuration.
- Incorrect Gauge Reading: Dashboard exhaust temperature gauge reads -40 °C or erratic values due to low-voltage signal from the sensor.
Probable Causes
- Shorted Sensor Wiring: Insulation chafed against chassis ground or exhaust pipe, creating a direct short to ground.
- Failed Sensor Element: Internal NTC thermistor cracked or shorted due to thermal shock from rapid cooling after regen.
- ECM Internal Fault: Pull-up resistor inside ECM damaged, causing signal voltage to stay below 0.2 V even with sensor disconnected.
- Connector Corrosion: Moisture ingress at the 2-pin Delphi or Deutsch connector creates a low-resistance path to ground.
Advanced Technical Analysis
The ECM monitors the exhaust temperature sensor signal pin (typically pin 15 on the 120-pin connector) through a 5 V pull-up resistor. A normal NTC sensor reads 3.5 V at 20 °C and 0.5 V at 800 °C. When the ECM detects voltage below 0.2 V for more than 2 seconds, it sets SPN 173 FMI 4. The diagnostic timer prevents false triggers from transient noise but confirms a hard short.
Electrically, a short-to-ground can occur in three places: sensor internal resistance < 10 Ω, harness resistance < 5 Ω to chassis, or ECM internal short. Using a digital multimeter, technicians measure resistance between the signal pin and ground. A reading below 100 Ω confirms a short. Disconnecting the sensor should raise the signal voltage to 5 V; if it remains low, the ECM or harness is at fault.
When this fault is active, the ECM enters a safety fallback: it substitutes a default exhaust temperature value of -40 °C, disables DPF regeneration, and activates a torque derate of 25% to 40%. The derate prevents uncontrolled thermal stress on the DOC and DPF. Some OEMs, like Mercedes-Benz, also disable EGR to protect the cooler from unmonitored exhaust gas temperatures.
Long-term prevention requires inspecting the sensor harness routing, especially near exhaust manifold flanges and DPF inlets. In workshops, this fault is frequently seen after a turbocharger replacement where the harness was not properly re-secured. Technicians should apply high-temperature loom (e.g., Techflex Thermo-Shield) and use dielectric grease in connectors. After repair, a test drive with active regen confirms the signal returns to normal (0.5–4.5 V).
Step-by-Step Troubleshooting Guide
- Visual Harness Check: Inspect wiring from sensor to ECM for melted insulation, pinch points, or contact with exhaust heat shields.
- Sensor Resistance Test: Disconnect sensor, measure resistance between pins. 20 °C should read 2.5–3.5 kΩ; short indicates failure.
- Signal Voltage Check: With sensor disconnected, measure ECM signal pin voltage. Should be 4.8–5.2 V. If low, repair harness short.
- ECM Pin-Out Test: Back-probe ECM connector while sensor connected. Voltage below 0.2 V confirms short; if above, replace sensor.