SPN 164 FMI 3 – Rail Pressure Sensor Circuit Failed High Fault Code (Symptoms, Causes & Fix)

Mar 20, 2026

The SPN 164 FMI 3 fault code indicates a 'Rail Pressure Sensor Circuit Failed High' condition in a diesel engine. This diagnostic trouble code suggests that the Engine Control Module (ECM) has detected a higher than expected voltage signal from the rail pressure sensor, which is critical for monitoring the fuel rail pressure in the fuel system.

What SPN 164 FMI 3 Means

The SPN 164 FMI 3 fault code is triggered when the ECM receives a signal from the rail pressure sensor that exceeds the expected range. This sensor is responsible for providing real-time data on the fuel pressure within the rail, which is crucial for optimal engine performance and fuel efficiency. A high signal typically indicates an electrical issue, such as a short to voltage, rather than a mechanical problem with the fuel system itself.

Common Causes of This Fault Code

Failed sensor
Wiring harness damage
Connector corrosion
ECM signal problems
Voltage reference faults

Symptoms Drivers May Notice

When the SPN 164 FMI 3 fault code is active, drivers may experience symptoms such as reduced engine performance, poor fuel economy, or even an engine derate condition. The Check Engine Light may illuminate, and there could be noticeable hesitation or rough idling.

How Technicians Diagnose This Fault

Technicians begin diagnosing the SPN 164 FMI 3 fault by performing a visual inspection of the rail pressure sensor and its associated wiring. They check for any physical damage, corrosion, or loose connections. Next, they use a multimeter to test the voltage and continuity of the sensor circuit, looking for shorts to voltage or open circuits. The technician will also verify the 5-volt reference circuit, if applicable, and compare the sensor output voltage to the manufacturer's specifications using diagnostic software. If necessary, the sensor may be swapped with a known good unit to determine if the fault persists.

Common Misdiagnosis

A common misdiagnosis is replacing the rail pressure sensor without thoroughly checking the wiring harness and connectors. Technicians may also mistakenly attribute the issue to a mechanical fuel system problem without verifying the electrical circuit. Additionally, overlooking potential ECU software updates or faults can lead to incorrect conclusions.

Can You Keep Driving?

While it may be possible to continue driving with the SPN 164 FMI 3 fault code active, it is not advisable. The fault can lead to engine derate or protection mode, significantly affecting performance and potentially causing further damage. It is best to address the issue promptly to avoid these complications.

Mechanic's Diagnostic Tip

When diagnosing SPN 164 FMI 3, always start with a thorough inspection of the wiring and connectors. Many issues stem from simple electrical faults that can be resolved without replacing expensive components. Ensure all connections are clean and secure before moving on to more complex diagnostics.

Frequently Asked Questions

Q: Can a faulty rail pressure sensor cause engine performance issues?
A: Yes, a faulty sensor can lead to incorrect fuel pressure readings, affecting engine performance and efficiency.

Q: Is it necessary to replace the ECU if this fault code appears?
A: Not necessarily. The ECU should only be considered after confirming that the sensor and wiring are functioning correctly.

Q: How can I prevent this fault code from occurring?
A: Regular maintenance, including inspecting and cleaning electrical connections, can help prevent issues related to the rail pressure sensor circuit.