Detroit Diesel · Intake Manifold Temperature SensorMedium

Detroit Diesel SPN 105 FMI 3: Intake Manifold Temp Circuit High

SPN 105 FMI 3 (FMI 3) — Intake Manifold 1 Temperature - Voltage Above Normal or Shorted to High Source

4.8/5· 128 reviews

Quick Answer

SPN 105 FMI 3 means the intake manifold temperature sensor circuit is reading a voltage above normal, consistent with an open circuit or short to power. It's a medium-severity electrical fault on DD engines with no derate, but it does affect cold-start fueling accuracy.

Overview: Detroit Diesel SPN 105 FMI 3

This code shows up regularly on DD13 and DD15 engines with high mileage harnesses. ProfessionalDieselRepair.com always starts with a pin test at the connector before assuming the sensor itself has failed, since a chafed wire near the valve cover is the more frequent culprit.

Repair Snapshot

Time

45 Minutes - 1.5 Hours

Difficulty

Beginner

Parts Cost

$40 - $160

Dealer Labor

$260+

Tools Required

  • Detroit Diesel Diagnostic Link (DDDL)
  • Jaltest Diagnostic Tool
  • Multimeter
  • Wiring Repair Kit

What SPN 105 FMI 3 Means

SPN 105 FMI 3 on Detroit Diesel (DD13/DD15/DD16) engines is an electrical circuit fault, not a genuine overheat condition - the ECM sees the intake manifold temperature sensor signal pinned high, consistent with an open circuit or short to a voltage source. There's no derate tied to this code, but the ECM runs on a default assumed air temperature until it's fixed.

Symptoms Drivers Notice

  • Check engine lamp illuminated with no drivability complaint
  • Possible slightly rich or lean cold-start fueling
  • No power loss under normal operation

Most Likely Root Causes

Chafed signal wire near the valve cover or turbo heat shield

Common

Heat cycling and vibration wear through insulation over time, letting the signal wire short against a grounded or powered surface.

Corroded or backed-out connector pin

Occasional

Moisture intrusion at the connector can corrode pins, creating an intermittent open that the ECM reads as a high-voltage fault.

Internally failed sensor element

Occasional

A cracked thermistor inside the sensor housing can present as an open circuit, mimicking the same fault signature.

Common Misdiagnoses (Don't Waste Parts)

Swapping the sensor without checking harness continuity

Why it happens: It's a fast parts-cannon move on a low-severity code that doesn't demand deep diagnosis.

Do this instead: Back-probe with the sensor disconnected first - a signal still pinned high confirms the harness, not the sensor, is at fault.

Master Tech Commentary

Mechanic's Notes

Shop tip: ProfessionalDieselRepair.com finds this is almost always a harness issue on DD platforms - the sensor connector sits close to engine heat and vibration sources. Pin-test before replacing the sensor to avoid an unnecessary parts swap.

Step-by-Step Diagnostic Procedure

  1. Step 1: Pull the code and freeze-frame data on Detroit Diesel Diagnostic Link (DDDL) to confirm intake manifold temp is pinned at an unrealistic high value.
  2. Step 2: Locate the sensor at the intake manifold, unplug the connector, and back-probe signal and ground with a multimeter, key on engine off.
  3. Step 3: A signal reading near 5V with the sensor disconnected confirms the fault is in the harness rather than the sensor itself.
  4. Step 4: Trace the signal wire for chafing near the valve cover, turbo heat shield, or engine harness routing points.
  5. Step 5: Test the sensor's resistance at the connector against DDC's published temperature/resistance chart to rule out an internally open sensor.
  6. Step 6: Repair any damaged wiring or replace the sensor as needed, then clear the code and confirm a plausible reading returns with the engine cold.

Live Data & Freeze-Frame Tips

With the sensor plugged in and the engine cold, intake manifold temperature should read close to ambient. A value stuck at the sensor's maximum output confirms an electrical fault rather than an actual temperature condition.

Safety Warnings

  • Allow the engine and turbo heat shield to cool before accessing the intake manifold sensor.
  • Disconnect the battery before repairing any chafed wiring near the valve cover.

When to Limp In vs Tow

No towing is required for this code alone - it's a low-severity electrical fault that doesn't affect safe driving.

Prevention Tips for Fleets & Owner-Operators

  • Secure and protect wiring harnesses routed near turbo heat shields during any engine service.
  • Inspect connectors for corrosion during scheduled PM intervals.
  • Use dielectric grease on connector pins when reassembling after service work.

Related Fault Codes

SPN 105 FMI 4

The opposite electrical fault - voltage below normal or shorted to ground, instead of a short to a power source.

SPN 172 FMI 3

Engine air inlet temperature circuit high - a related sensor measuring air temp before the turbo instead of after the charge air cooler.

Frequently Asked Questions

Can I drive with Detroit Diesel SPN 105 FMI 3 active?

Yes, this code doesn't trigger a derate or shutdown. It's safe to keep driving while you schedule the repair.

How do I reset this code without a scanner?

There's no reliable manual reset. Use DDDL, Jaltest, or a comparable tool to clear the code once the wiring or sensor repair is verified.

Does this code cause a derate countdown?

No, this is a low-severity sensor circuit fault and does not trigger an emissions-related derate countdown.

Is the sensor usually bad, or is it the wiring?

The wiring harness is the more common culprit on DD platforms - a chafed signal wire near the valve cover shorting to power is more frequent than an actually failed sensor.

Will this affect fuel economy?

It can have a minor effect on cold-start fueling accuracy since the ECM defaults to an assumed temperature, but overall fuel economy impact is minimal.

Where exactly is this sensor located?

It's mounted directly on the intake manifold, downstream of the charge air cooler - don't confuse it with the air inlet temperature sensor located upstream near the turbo.