Kenworth · Engine Air Inlet Temperature SensorMedium

Kenworth SPN 172 FMI 4: Diagnosing a Low Intake Air Temperature Reading

SPN 172 FMI 4 (FMI 4) — Engine Air Inlet Temperature - Voltage Below Normal, Or Shorted To Low Source

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Quick Answer

SPN 172 FMI 4 on PACCAR MX-13 engines means the intake air temperature sensor circuit is reading below normal, consistent with a short to ground. It's a medium-severity fault that affects fueling and boost compensation strategy until it's fixed.

Overview: Kenworth SPN 172 FMI 4

The engine air inlet (intake) temperature sensor feeds the ECM's fueling and boost strategy, so when SPN 172 FMI 4 shows the circuit shorted low, the ECM ends up compensating with conservative defaults rather than accurate data. ProfessionalDieselRepair.com typically finds this comes down to connector corrosion or a chafed wire more often than a genuinely failed sensor - the sensor sits in the intake tract where road grime and moisture accumulate at the connector over time.

Repair Snapshot

Time

0.5 - 2 Hours

Difficulty

Beginner

Parts Cost

$25 - $110

Dealer Labor

$180+

Tools Required

  • PACCAR Davie4
  • Multimeter
  • Diesel Laptops Scan Tool
  • Wire Repair Kit

What SPN 172 FMI 4 Means

SPN 172 FMI 4 means the engine air inlet temperature sensor circuit is reading voltage below normal, consistent with a short to ground or an open reference. On a PACCAR MX-13 in a Kenworth, this is a medium-severity fault - the ECM leans on this reading for fueling and boost compensation, so a wrong value can affect performance and cause the ECM to default to a conservative fallback strategy.

Symptoms Drivers Notice

  • Slightly reduced performance as the ECM defaults to a conservative fueling map
  • Amber Check Engine lamp
  • Occasional rough cold starts if the sensor is defaulting to an unrealistic value
  • No dramatic drivability change in most cases - it's primarily a compensation/derate-avoidance strategy issue

Most Likely Root Causes

Corroded or moisture-damaged sensor connector

Common

The connector's exposed location in the intake tract makes it prone to moisture intrusion, which can create a low-resistance short path.

Chafed wiring shorting the signal to ground

Occasional

Harness rubbing against intake piping or a bracket can wear through insulation and short the signal circuit directly to ground.

Failed sensor element

Occasional

Internal sensor failure can produce a zero-resistance or shorted reading independent of external wiring condition.

Common Misdiagnoses (Don't Waste Parts)

Replacing the sensor without checking connector condition first

Why it happens: It's the fastest fix to attempt, so techs often skip straight to a swap - a small parts-cannon habit that misses an easy connector cleaning.

Do this instead: Clean and inspect the connector, then check resistance across the sensor before replacing it outright.

Master Tech Commentary

Mechanic's Notes

Shop tip: a shorted-low reading almost always means the sensor is pegged at an extreme cold value - check that number on the scan tool before you even pull the sensor. ProfessionalDieselRepair.com finds a corroded connector pin more often than an actual failed sensor element on this circuit.

Step-by-Step Diagnostic Procedure

  1. Step 1: Pull the code with PACCAR Davie4 and confirm SPN 172 FMI 4 is active; note the reported intake air temperature value, which is likely pegged at an unrealistic extreme (very cold reading).
  2. Step 2: Locate the air inlet temperature sensor, typically in the intake tract near the turbo inlet or intercooler piping, and inspect the connector for corrosion or looseness.
  3. Step 3: Disconnect the sensor and measure resistance across its terminals, comparing against the resistance-vs-temperature spec - a reading of zero ohms or an open circuit confirms sensor or wiring failure.
  4. Step 4: With the sensor disconnected, check the signal wire for a short to ground back at the harness using a multimeter - continuity to ground with the ECM disconnected confirms a chafed wire.
  5. Step 5: Repair the wiring if a short is found, or replace the sensor if it fails the resistance test, then clear the code.
  6. Step 6: Verify the reported intake air temperature now matches ambient conditions reasonably closely before returning the truck to service.

Live Data & Freeze-Frame Tips

Watch the reported intake air temperature on Davie4 - a value pegged at an extreme cold reading (often near or below the sensor's minimum measurable range) with no correlation to ambient conditions confirms the circuit fault. Compare it against the ambient air temperature sensor reading for a sanity check.

Safety Warnings

  • Allow the intake system to cool if the engine has been running before disconnecting sensors near hot components.
  • Disconnect the battery before repairing any chafed wiring to prevent accidental shorts.

When to Limp In vs Tow

Towing isn't necessary for this fault - the truck remains safely drivable with a default fueling strategy while the sensor circuit is repaired.

Prevention Tips for Fleets & Owner-Operators

  • Inspect the intake air temperature sensor connector for moisture during routine PM services.
  • Route wiring away from sharp edges in the intake tract during any related service work.
  • Apply dielectric grease to the connector when reassembling after any inspection.

Related Fault Codes

SPN 172 FMI 3

Same sensor circuit flagged as voltage above normal instead of below - opposite failure mode, likely a short-to-power rather than short-to-ground.

SPN 105 FMI 4

Intake manifold temperature sensor circuit fault - a related but physically separate sensor with a similar diagnostic approach.

SPN 1127 FMI 18

Low boost pressure - can occur alongside intake temperature faults if the intake tract has a broader air-path issue.

Frequently Asked Questions

Can I drive with SPN 172 FMI 4 active?

Yes, the truck remains drivable using a default fueling and boost strategy, though performance may be slightly reduced. Repair it when convenient rather than as an emergency.

How do I reset SPN 172 FMI 4 without a scan tool?

You need Davie4 or a comparable scan tool to clear the stored code after the sensor circuit is repaired; there's no manual reset from the dash.

Is a bad sensor or bad wiring more likely to cause this code?

Wiring and connector corrosion cause this fault more often than a genuinely failed sensor element - check the connector first.

Will this code trigger an engine derate?

No, this specific fault typically results in the ECM using a default fallback value rather than triggering an active power derate.

Where exactly is this sensor located?

It's mounted in the intake tract, usually near the turbo inlet or intercooler piping, positioned to measure the temperature of incoming combustion air.

Can extreme cold weather cause a false version of this code?

No - the sensor circuit distinguishes between a genuinely cold reading and an electrical short. A true short-to-ground condition reads well beyond any realistic ambient temperature.