SPN 164 FMI 20 – Rail Pressure Sensor, Signal Drift (High) Fault Code (Symptoms, Causes & Fix)

Mar 20, 2026

The SPN 164 FMI 20 fault code indicates a high signal drift detected by the rail pressure sensor in a diesel engine. This diagnostic trouble code suggests that the Engine Control Module (ECM) is receiving a signal from the rail pressure sensor that is outside the expected range, potentially due to sensor malfunction or electrical issues.

What SPN 164 FMI 20 Means

The SPN 164 FMI 20 fault code is related to the fuel rail pressure sensor, which monitors the pressure within the fuel rail. The ECM uses this data to ensure optimal fuel delivery to the engine. When the ECM detects a signal drift that is higher than expected, it flags this fault code, indicating a potential issue with the sensor or its associated circuitry.

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 20 fault code is active, drivers may notice symptoms such as reduced engine performance, increased fuel consumption, or the engine entering a derate mode. In some cases, the check engine light may illuminate, alerting the driver to a potential issue.

How Technicians Diagnose This Fault

Technicians begin diagnosing the SPN 164 FMI 20 fault code with a visual inspection of the wiring harness and connectors for any signs of damage or corrosion. Next, they verify the sensor's power supply and ground circuits using a multimeter. Checking the 5-volt reference circuit is crucial to ensure proper sensor operation.

Using diagnostic software, technicians compare the actual rail pressure readings with the commanded values to identify discrepancies. If necessary, they perform a rail pressure sensor calibration or reset. Additionally, inspecting the fuel rail pressure relief valve for proper operation is essential to rule out mechanical issues.

Common Misdiagnosis

A common misdiagnosis with the SPN 164 FMI 20 fault code is replacing the rail pressure sensor without thoroughly checking the wiring integrity. Technicians may also overlook ECU software updates or calibration issues, leading to unnecessary part replacements. Misinterpreting sensor drift as a mechanical fuel system issue is another frequent error.

Can You Keep Driving?

While it may be possible to continue driving with the SPN 164 FMI 20 fault code active, it is not advisable. The fault can lead to engine derate or protection mode, reducing performance and potentially causing further damage. Addressing the issue promptly is recommended to maintain optimal engine function.

Mechanic's Diagnostic Tip

When diagnosing the SPN 164 FMI 20 fault code, always verify the integrity of the wiring harness and connectors before replacing the sensor. This step can prevent unnecessary part replacements and ensure a more accurate diagnosis.

Frequently Asked Questions

Q: What does SPN 164 FMI 20 indicate?
A: It indicates a high signal drift detected by the rail pressure sensor, suggesting a potential issue with the sensor or its circuitry.

Q: Can a faulty rail pressure sensor cause engine derate?
A: Yes, a faulty sensor can lead to engine derate as the ECM may not receive accurate pressure readings, affecting fuel delivery.

Q: Is it safe to drive with the SPN 164 FMI 20 fault code?
A: It is not recommended to drive with this fault code active, as it can lead to reduced performance and potential engine damage.