SPN 3464 FMI 31 – H Bridge 1 IAT Circuit Open Load Fault Code (Symptoms, Causes & Fix)

Sep 22, 2026

The SPN 3464 FMI 31 fault code is a diesel engine diagnostic trouble code indicating an open load condition in the H Bridge 1 Intake Air Temperature (IAT) circuit. This code is triggered when the Engine Control Module (ECM) detects an open circuit or lack of continuity in the IAT sensor circuit, which is crucial for monitoring the temperature of the air entering the engine.

What SPN 3464 FMI 31 Means

The SPN 3464 FMI 31 fault code specifically points to an issue with the H Bridge 1 IAT circuit, where the ECM identifies an open load condition. This means there is a disruption in the electrical path, preventing the IAT sensor from sending accurate temperature data to the ECM. The IAT sensor plays a vital role in adjusting fuel delivery and engine timing based on the air temperature, so any fault here can affect engine performance.

Common Causes of This Fault Code

Open circuit in the IAT sensor wiring
Faulty IAT sensor
Corroded or damaged connectors
ECM signal problems
Voltage reference faults

Symptoms Drivers May Notice

When the SPN 3464 FMI 31 fault code is active, drivers may notice symptoms such as reduced engine performance, poor fuel economy, or erratic idling. In some cases, the engine may enter a derate mode to protect itself from potential damage due to incorrect air temperature readings.

How Technicians Diagnose This Fault

Technicians begin diagnosing the SPN 3464 FMI 31 fault code with a thorough visual inspection of the IAT sensor and its wiring harness for any signs of damage or corrosion. Next, they use a multimeter to check for continuity in the IAT sensor circuit, ensuring there are no open circuits. The resistance of the IAT sensor is measured to verify it is within the manufacturer's specifications. Connectors are inspected for proper contact and corrosion, and an oscilloscope may be used to check the signal from the IAT sensor to the ECM. Throughout this process, technicians ensure the power supply and ground circuits are functioning correctly.

Common Misdiagnosis

A common misdiagnosis with the SPN 3464 FMI 31 fault code is replacing the IAT sensor without thoroughly checking the wiring harness for open circuits. Technicians may also mistakenly assume the ECM is faulty without verifying the integrity of the sensor and wiring. Overlooking connector corrosion or damage is another frequent error.

Can You Keep Driving?

While it may be possible to continue driving with the SPN 3464 FMI 31 fault code active, it is not recommended. The engine may enter a derate mode, leading to reduced power and efficiency. Addressing the issue promptly can prevent further engine performance issues and potential damage.

Mechanic's Diagnostic Tip

When diagnosing the SPN 3464 FMI 31 fault code, always start with a thorough inspection of the wiring and connectors. Many issues stem from simple problems like corrosion or loose connections, which can be resolved without replacing expensive components.

Frequently Asked Questions

What does SPN 3464 FMI 31 indicate? It indicates an open load condition in the H Bridge 1 IAT circuit, affecting the sensor's ability to communicate with the ECM.
Can a faulty IAT sensor cause engine performance issues? Yes, a faulty IAT sensor can lead to incorrect air temperature readings, affecting fuel delivery and engine timing.
Is it safe to drive with this fault code? While you may be able to drive, it's not advisable as it can lead to reduced engine performance and efficiency.