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SPN 171 FMI 20 – Ambient Temperature Sensor Signal Not Plausible Fault Code (Symptoms, Causes & Fix)

The SPN 171 FMI 20 fault code is a diesel engine diagnostic trouble code indicating that the ambient temperature sensor signal is not plausible. This means the Engine Control Module (ECM) is receiving a signal from the ambient temperature sensor that doesn't align with expected parameters, suggesting a potential issue with the sensor or its circuit.

What SPN 171 FMI 20 Means

The SPN 171 FMI 20 fault code specifically relates to the ambient temperature sensor, which provides the ECM with data about the outside air temperature. The ECM uses this information to adjust engine parameters for optimal performance. When the signal from this sensor is deemed implausible, it indicates a discrepancy between the actual and expected sensor readings, potentially due to electrical or sensor faults.

Common Causes of This Fault Code

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

Symptoms Drivers May Notice

When the SPN 171 FMI 20 fault code is active, drivers might notice inaccurate temperature readings on the dashboard. This can lead to improper engine management adjustments, potentially affecting fuel efficiency and engine performance. However, this fault typically doesn't cause immediate drivability issues.

How Technicians Diagnose This Fault

Technicians begin diagnosing the SPN 171 FMI 20 fault by performing a visual inspection of the ambient temperature sensor and its connector for any signs of physical damage or debris. Next, they check the sensor connector for corrosion, bent pins, or poor connections. Using a multimeter, they perform a continuity test on the wiring harness from the sensor to the ECM to identify any open or short circuits.

Technicians then measure the sensor's resistance with a multimeter and compare it to the manufacturer's specifications. They also verify the sensor's installation and placement to ensure it's accurate. Finally, they check for any ECM software updates or recalibration requirements that might resolve the issue.

Common Misdiagnosis

A common misdiagnosis with the SPN 171 FMI 20 fault code is replacing the ambient temperature sensor without first checking the wiring harness and connectors for issues. Technicians might also overlook connector problems due to corrosion or poor contact, leading to unnecessary sensor replacements.

Can You Keep Driving?

While the SPN 171 FMI 20 fault code doesn't typically cause immediate drivability issues, it's advisable to address it promptly. Ignoring the fault can lead to inefficient engine performance and potential long-term engine management issues. However, it usually doesn't trigger an engine derate or protection mode.

Mechanic's Diagnostic Tip

Always verify the integrity of the wiring harness and connectors before replacing the ambient temperature sensor. Many issues stem from poor electrical connections rather than sensor failure, so thorough electrical testing can prevent unnecessary part replacements.

Frequently Asked Questions

Q: What tools are needed to diagnose SPN 171 FMI 20?
A: A multimeter and diagnostic software are essential for testing the sensor and verifying ECM data.

Q: Can a software update fix SPN 171 FMI 20?
A: In some cases, an ECM software update or recalibration may resolve the fault if it's related to software glitches.

Q: Is it safe to drive with SPN 171 FMI 20 active?
A: While it's generally safe to drive, addressing the fault promptly is recommended to maintain optimal engine performance.

Other FMI Codes for SPN 171