Freightliner · Engine Exhaust Gas Temperature SensorMedium

Freightliner SPN 173 FMI 3: Exhaust Gas Temp Sensor High

SPN 173 FMI 3 (FMI 3) — Engine Exhaust Gas Temperature - Voltage Above Normal

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Quick Answer

SPN 173 FMI 3 means an exhaust gas temperature sensor circuit on a Freightliner is reading voltage above normal, typically an open sensor or heat-damaged wire. It's Medium severity and can affect regeneration timing, though the engine keeps running normally.

Overview: Freightliner SPN 173 FMI 3

Exhaust gas temperature sensors are scattered throughout the aftertreatment system on modern Freightliners to give the ECM real-time visibility into exhaust heat at multiple points. SPN 173 FMI 3 flags an open circuit condition on one of these sensors, and while it won't cause any immediate drivability problem, ProfessionalDieselRepair.com always recommends addressing it promptly since it can quietly interfere with regeneration timing decisions the ECM needs to keep the DPF and SCR system healthy.

Repair Snapshot

Time

1 - 2 Hours

Difficulty

Intermediate

Parts Cost

$70 - $230

Dealer Labor

$380+

Tools Required

  • Detroit Diesel Diagnostic Link (DDDL)
  • Multimeter
  • Diesel Laptops Diagnostic Software
  • Heat-Resistant Gloves

What SPN 173 FMI 3 Means

SPN 173 FMI 3 means an exhaust gas temperature sensor circuit is reading voltage above the expected range, which the ECM reads as an open circuit or a signal pinned high. EGT sensors feed both turbo protection strategy and aftertreatment regeneration timing, so this fault can interfere with regen behavior and cause the ECM to run more conservative timing. The engine keeps running normally, but the fault should not be ignored long term.

Symptoms Drivers Notice

  • Amber MIL illuminated
  • Possible DPF regeneration timing changes or hesitancy to initiate regen
  • No immediate drivability change in most cases
  • Slightly reduced fuel economy on some calibrations as the ECM runs conservative defaults

Most Likely Root Causes

Heat-damaged sensor wiring near the exhaust system

Common

Sustained high exhaust temperatures gradually crack wire insulation, eventually breaking the conductor and opening the circuit.

Corroded or loose sensor connector

Common

Heat cycling and road grime degrade connector seals over time, corroding pin contacts and creating an intermittent or open connection.

Failed sensor element

Occasional

The thermocouple or thermistor element inside the sensor can fail from repeated extreme heat cycling.

Physical damage from a recent exhaust or turbo repair

Rare

Sensor wiring or connectors can be accidentally damaged during nearby component service if care isn't taken during reassembly.

Common Misdiagnoses (Don't Waste Parts)

Replacing the turbocharger due to perceived overheating concerns

Why it happens: A parts-cannon mindset assumes any exhaust temperature fault means the turbo itself is failing or overheating.

Do this instead: Verify the sensor circuit is actually open before assuming any real overheat condition exists, since the fault is about the signal, not necessarily the actual temperature.

Skipping the wiring check because the sensor tests fine off the truck

Why it happens: Techs assume the sensor must be bad once removed without checking harness continuity separately.

Do this instead: Test reference voltage and ground at the harness connector independently of the sensor before concluding the sensor is at fault.

Ignoring the fault since there's no obvious drivability change

Why it happens: Without a noticeable symptom, this code often gets deprioritized.

Do this instead: Address it promptly since prolonged operation without accurate EGT data can affect regeneration reliability and long-term aftertreatment health.

Master Tech Commentary

Mechanic's Notes

Shop tip from ProfessionalDieselRepair.com: EGT sensor wiring lives in one of the harshest thermal environments on the truck, so a cracked insulation jacket right at the heat shield boundary is far more common than an actual failed sensor element. Inspect the last several inches of harness closest to the exhaust before ordering a new sensor.

Step-by-Step Diagnostic Procedure

  1. Step 1: Connect a Freightliner-compatible diagnostic tool and confirm SPN 173 FMI 3, noting which specific EGT sensor location is affected if the truck has multiple.
  2. Step 2: Inspect the affected sensor connector for corrosion, and check the wiring for heat damage near the exhaust piping and turbo.
  3. Step 3: Unplug the sensor and verify the expected reference voltage and ground are present at the harness side.
  4. Step 4: Check sensor resistance with a multimeter against spec, since an open EGT sensor will read infinite resistance (OL), consistent with a voltage-above-normal fault.
  5. Step 5: Replace the sensor or repair the harness as needed, clear the code, and monitor regeneration behavior on the next drive cycle.

Live Data & Freeze-Frame Tips

Compare the affected EGT sensor reading against adjacent temperature sensors in the exhaust path using your diagnostic tool - a stuck-high or unrealistic flat value confirms an open circuit rather than an actual temperature anomaly.

Safety Warnings

  • Allow the exhaust system to cool completely before working near EGT sensors - temperatures can remain dangerously hot well after shutdown.
  • Wear heat-resistant gloves when handling sensors and connectors near the exhaust piping.
  • Support the vehicle properly on a lift or jack stands when accessing components underneath.
  • Avoid sensor service immediately after a regeneration event due to extreme residual heat.

When to Limp In vs Tow

Towing is not necessary for this code. Address it promptly to maintain reliable regeneration timing and avoid any secondary aftertreatment complications down the road.

Prevention Tips for Fleets & Owner-Operators

  • Inspect EGT sensor wiring for heat damage during routine PM inspections.
  • Ensure heat shields are properly reinstalled after any exhaust or turbo service.
  • Address minor sensor codes early before a chafe or crack point worsens into a full open circuit.
  • Keep a record of which EGT sensor locations have needed replacement to spot recurring heat-damage patterns.

Related Fault Codes

SPN 3242 FMI 3

DPF intake gas temperature voltage high, a related exhaust temperature sensor feeding the same regeneration strategy.

SPN 4360 FMI 3

SCR catalyst intake gas temperature voltage high, another temperature input in the same exhaust heat monitoring network.

SPN 641 FMI 7

VGT actuator mechanical fault, worth checking since abnormal EGT readings can sometimes accompany turbo control issues.

Frequently Asked Questions

Can I drive with SPN 173 FMI 3 active?

Yes, the engine runs normally with this code, though regeneration timing decisions may be affected since the ECM is missing accurate data from this temperature sensor.

How do I reset this code without a scanner?

You need a scan tool to properly diagnose and confirm the repair, since simply clearing the code without fixing the open circuit will result in the fault returning.

Does this code affect fuel economy?

Some ECM calibrations run more conservative default strategies when a temperature sensor is unavailable, which can slightly affect fuel economy until the sensor is repaired.

Is this the same sensor used for turbo protection?

Depending on its location, some exhaust gas temperature sensors do feed turbo protection strategies in addition to aftertreatment regeneration timing, so check your specific sensor location in the wiring diagram.

How expensive is this sensor to replace?

EGT sensors are generally inexpensive, typically well under $200 including labor, making this one of the more affordable aftertreatment-related repairs.

Can heat exposure alone cause this fault without any accident or damage?

Yes, normal long-term exposure to high exhaust temperatures is enough to eventually crack wire insulation and cause an open circuit, even without any collision or impact damage.