PACCAR · Intake Manifold Temperature SensorMedium

PACCAR SPN 105 FMI 4: Diagnosing a Low Intake Manifold Temperature Reading

SPN 105 FMI 4 (FMI 4) — Engine Intake Manifold 1 Temperature - Voltage Below Normal, Or Shorted To Low Source

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Quick Answer

SPN 105 FMI 4 on PACCAR MX-13 engines means the intake manifold temperature sensor circuit is reading below normal, consistent with a short to ground. It's a medium-severity fault that pushes the ECM toward conservative default fueling until repaired.

Overview: PACCAR SPN 105 FMI 4

Intake manifold temperature works alongside boost pressure to help the ECM fine-tune fueling and EGR strategy, and SPN 105 FMI 4 means that input has dropped out with a shorted-low reading. ProfessionalDieselRepair.com typically finds this comes down to a corroded or contaminated connector at the sensor's mounting point on the manifold itself, since it's directly exposed to engine bay conditions including oil mist and moisture 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 105 FMI 4 Means

SPN 105 FMI 4 means the intake manifold temperature sensor circuit is reading voltage below normal, consistent with a short to ground. On a PACCAR MX-13, this is a medium-severity fault - the ECM uses this reading alongside boost pressure for fueling and EGR compensation, so a faulted circuit pushes the engine toward a conservative default strategy until it's fixed.

Symptoms Drivers Notice

  • Slightly reduced performance from conservative default fueling
  • Amber Check Engine lamp
  • Reported intake manifold temperature pegged at an unrealistic cold value
  • No dramatic drivability complaint in most cases

Most Likely Root Causes

Corroded or contaminated sensor connector

Common

The sensor's location directly on the intake manifold exposes its connector to oil mist, moisture, and heat cycling that accelerates corrosion.

Chafed wiring shorting the signal to ground

Occasional

Harness routing near the manifold can wear through insulation over time, creating a short-to-ground path.

Failed sensor element

Occasional

Internal sensor failure can independently produce a shorted-low reading, though it's less common than connector or wiring issues in this location.

Common Misdiagnoses (Don't Waste Parts)

Replacing the sensor without cleaning the connector first

Why it happens: It's a quick swap that seems like the obvious fix - a minor parts-cannon habit that skips an easy first step.

Do this instead: Clean and inspect the connector thoroughly, checking for contamination, before replacing the sensor outright.

Master Tech Commentary

Mechanic's Notes

Shop tip: this sensor threads directly into the intake manifold, so check for oil or coolant contamination at the connector too, not just corrosion from moisture. ProfessionalDieselRepair.com always cleans and inspects the whole connector area thoroughly before assuming the sensor element itself has failed.

Step-by-Step Diagnostic Procedure

  1. Step 1: Pull the fault with PACCAR Davie4 and confirm SPN 105 FMI 4 is active; check the reported intake manifold temperature value, which will likely be pegged at an extreme cold reading.
  2. Step 2: Locate the sensor, typically threaded into the intake manifold itself, and inspect the connector for corrosion or a pinched wire nearby.
  3. Step 3: Disconnect the sensor and check the signal wire for continuity to ground with the harness isolated from the ECM - any unexpected continuity confirms a short.
  4. Step 4: Test the sensor's resistance at a known temperature and compare against the manufacturer's resistance-versus-temperature chart to rule the sensor element in or out.
  5. Step 5: Repair any chafed wiring found, or replace the sensor if it fails the resistance test.
  6. Step 6: Clear the code and confirm the reported temperature climbs realistically as the engine warms up during a test run before returning the truck to service.

Live Data & Freeze-Frame Tips

Watch the reported intake manifold temperature on Davie4 - a value pegged at an extreme cold reading with no correlation to actual engine warm-up confirms the circuit fault. Compare it against the intake air temperature sensor (SPN 172) reading for a quick sanity check between related temperature inputs.

Safety Warnings

  • Allow the intake manifold to cool if the engine has been running before disconnecting the sensor.
  • Disconnect the battery before repairing any chafed wiring near the manifold area.

When to Limp In vs Tow

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

Prevention Tips for Fleets & Owner-Operators

  • Inspect the intake manifold temperature sensor connector for contamination during routine PM services.
  • Address any oil leaks near the intake manifold promptly to prevent connector fouling.
  • Apply dielectric grease to the connector when reassembling after any inspection.

Related Fault Codes

SPN 105 FMI 3

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

SPN 172 FMI 4

Engine air inlet temperature circuit fault - a related but physically separate temperature sensor with a similar diagnostic approach.

SPN 1127 FMI 18

Low turbo boost pressure - not directly related, but both are intake-side parameters worth checking together if broader airflow issues are suspected.

Frequently Asked Questions

Can I drive with SPN 105 FMI 4 active?

Yes, the truck remains drivable using a default fueling strategy, though performance may be slightly reduced. Schedule the repair at your next convenient service.

How do I reset SPN 105 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 wiring or the sensor itself more likely to be the cause?

Connector corrosion or contamination is the more common cause given this sensor's exposed location directly on the intake manifold - check that before replacing the sensor.

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 threaded directly into the intake manifold, positioned to measure the temperature of air just before it enters the cylinders.

Can I test this sensor with a multimeter myself?

Yes, disconnect it and check resistance at a known ambient temperature, comparing against the manufacturer's resistance-versus-temperature chart to determine if the sensor element itself is faulty.