SPN 171 - FMI 4 (Fault Code 256)

SPN 171 - FMI 4 (Fault Code 256)
ISB - Barometric Pressure Sensor Circuit

Fault Code: 256
PID(P), SID(S): P171
SPN: 171
FMI: 4/4
ISC and ISL - Barometric Pressure Sensor Circuit

REASON: Ambient Air Temperature Sensor 1 Circuit - Voltage Below Normal or Shorted to Low Source. Low signal voltage detected at the ambient air temperature circuit.

EFFECT: None on performance.

Circuit Description:
The ambient air temperature sensor is a variable resistor sensor and is used to measure the temperature of the air entering the turbocharger compressor inlet of the engine. The electronic control module (ECM) supplies 5 volts to the ambient air temperature signal circuit. The ECM monitors the change in voltage caused by changes in the resistance of the sensor to determine the ambient air temperature.

Component Location:
The ambient air temperature sensor is located in the air intake.

Conditions for Running the Diagnostics:
This diagnostic runs continuously when the keyswitch is in the ON position.

Conditions for Setting the Fault Codes:
The ECM detects that the ambient air temperature signal voltage is less than 0.12-VDC for more than 1 second.

Action Taken When the Fault Code is Active:
• The ECM illuminates the amber CHECK ENGINE light immediately when the diagnostic runs and fails.
• A default value for the ambient air temperature reading is used.

Conditions for Clearing the Fault Code:
The ECM will turn off the amber CHECK ENGINE light immediately after the diagnostic runs and passes.

Shop Talk:
  • Verify the electronic control module (ECM) calibration is correct. Check the calibration revision history found on QuickServe® Online for applicable fixes to the calibration stored in the ECM. If necessary, recalibrate the ECM.
  • The ambient air temperature sensor shares return wires in the engine harness with other sensors. Opens and shorts in the engine harness can cause multiple fault codes to be active. Check fault codes with multiple counts first.

Possible causes of this fault code include:
• Signal circuit shorted to ground in the harness
• Signal circuit shorted to return or ground in the sensor.

Read More:
SPN 98 - FMI 1 (Faut Code 253)
SPN 171 - FMI 3 (Fault Code 249)


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