SPN 108 FMI 13 – Ambient Air Pressure Signal Not Available via CAN

This fault code indicates that the engine control unit (ECU) is not receiving a valid ambient air pressure signal via the Controller Area Network (CAN). This issue can lead to poor engine performance and, in some cases, derate behavior if not addressed promptly.

What This Code Means

SPN 108 represents the ambient air pressure sensor, which is critical for calculating air density and optimizing fuel delivery. FMI 13 indicates that the signal from this sensor is either missing or invalid, suggesting a potential issue with the sensor itself, its wiring, or the ECU's ability to interpret the signal.

Why It Happens

There are several reasons why this code may trigger. Common causes include:

• Mechanical Failures: Issues with the ambient air pressure sensor, such as physical damage or internal failure, can prevent proper signal transmission.
• Electrical Failures: Wiring problems, such as shorts, opens, or corrosion in connectors, can disrupt the signal path between the sensor and the ECU.
• Scaling Issues: Sensors may require calibration or scaling adjustments if they have been replaced or have drifted from their original settings.
• Restriction Testing: Blockages in the air intake system can lead to erroneous readings from the sensor.

How The Truck Typically Behaves

When SPN 108 FMI 13 is active, the truck may exhibit several symptoms, including:

• Reduced Power: The engine may enter a derate mode, limiting power output to protect itself from potential damage.
• Increased Fuel Consumption: Inaccurate air pressure readings can disrupt the air-fuel ratio, leading to inefficient combustion.
• Check Engine Light: The dashboard will typically display a check engine light, indicating a fault that needs attention.

Step-by-Step Diagnostic Approach

1. Scan for Codes: Begin by using a diagnostic scanner to confirm SPN 108 FMI 13 is present and check for any related fault codes.
2. Visual Inspection: Inspect the ambient air pressure sensor and its wiring for any signs of damage, corrosion, or loose connections.
3. Testing the Sensor: Use a multimeter to measure the voltage output of the ambient air pressure sensor. The expected output should vary based on atmospheric conditions; refer to manufacturer specifications for scaling.
4. Air System Checks: Perform a thorough inspection of the air intake system for blockages or restrictions that could affect sensor readings.
5. Wiring Continuity Check: Test the wiring between the sensor and the ECU for continuity and resistance. Look for shorts or opens that may indicate a failure.
6. ECU Functionality: If the sensor and wiring are functional, consider testing the ECU for proper operation and signal interpretation.

Can You Keep Driving?

It is not advisable to continue driving with SPN 108 FMI 13 active. The potential for derate behavior poses a risk of being stranded or causing further damage to the engine. Addressing the issue promptly will ensure optimal performance and reliability.

Frequently Asked Questions

• What should I do if the ambient air pressure sensor is faulty?
  Replace the sensor and perform a recalibration if necessary.
• How can I prevent this code from appearing?
  Regular maintenance, including visual inspections of wiring and sensors, can help prevent issues.
• Is there a specific model affected more than others?
  While SPN 108 FMI 13 can occur in various models, Freightliner Cascadia with DD15 engines may show this code due to their reliance on precise air pressure readings.