SPN 175 FMI 20 – Engine Oil Temperature Sensor Signal Not Plausible Fault Code (Symptoms, Causes & Fix)
The SPN 175 FMI 20 fault code is a diesel engine fault code indicating that the engine oil temperature sensor signal is not plausible. This diagnostic trouble code suggests that the Engine Control Module (ECM) is receiving a signal from the engine oil temperature sensor that does not align with expected parameters, potentially due to electrical issues or sensor failure.
What SPN 175 FMI 20 Means
The SPN 175 FMI 20 fault code specifically relates to the engine oil temperature sensor circuit. The ECM interprets this fault when the sensor's signal is outside the expected range, which could be due to a faulty sensor, wiring issues, or an ECM malfunction. This code indicates an electrical fault rather than a mechanical one.
Common Causes of This Fault Code
- Failed engine oil temperature sensor
- Wiring harness damage
- Connector corrosion
- ECM signal problems
- Voltage reference faults
Symptoms Drivers May Notice
When the SPN 175 FMI 20 fault code is active, drivers may notice the engine warning light illuminated on the dashboard. In some cases, the engine may enter a derate mode to protect itself from potential damage due to incorrect oil temperature readings.
How Technicians Diagnose This Fault
Technicians begin diagnosing this fault by performing a visual inspection of the wiring harness and connectors for any signs of damage or corrosion. Next, they check the battery voltage and overall system voltage to ensure proper electrical supply. Using a multimeter, they verify the sensor's power supply and ground circuits, followed by checking the 5-volt reference circuit if applicable.
Technicians then perform a resistance check on the engine oil temperature sensor and use a multimeter to ensure proper voltage and ground at the sensor connector. An oscilloscope may be used to verify the sensor signal is within expected parameters. Finally, they check for any ECU software updates or reflash if necessary.
Common Misdiagnosis
A common misdiagnosis is replacing the engine oil temperature sensor without thoroughly checking the wiring harness and connectors. Technicians may also mistakenly assume a mechanical issue with the engine oil system without verifying the electrical components. Overlooking potential ECU-related issues or necessary software updates is another frequent error.
Can You Keep Driving?
While it may be possible to continue driving with the SPN 175 FMI 20 fault code active, it is not advisable. The engine may enter a derate mode, reducing performance to prevent damage. It is best to address the issue promptly to avoid further complications.
Mechanic's Diagnostic Tip
Always verify the integrity of the wiring harness and connectors before replacing the engine oil temperature sensor. Many issues arise from poor electrical connections rather than sensor failure. This step can save time and prevent unnecessary part replacements.
Frequently Asked Questions
Q: What does SPN 175 FMI 20 indicate?
A: It indicates an implausible signal from the engine oil temperature sensor, suggesting an electrical issue.
Q: Can a faulty sensor cause engine derate?
A: Yes, a faulty sensor reading can cause the engine to enter derate mode to protect itself.
Q: Should I replace the sensor immediately?
A: No, perform thorough electrical testing first to confirm the sensor is the issue before replacing it.
