SPN 171 - FMI 4 (Fault Code 256)
Saturday, July 14, 2018
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SPN171 FMI4
Ambient Air Temperature Sensor 1 Circuit - Voltage Below Normal or Shorted to Low Source
CODES:
Fault Code: 256
PID(P), SID(S): P171
SPN: 171
FMI: 4/4
Lamp: Amber
REASON: Low voltage detected at 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 ambient air. The engine control module (ECM) uses this temperature reading to optimize engine performance. The 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 mounting location for the ambient air temperature sensor is OEM dependent. Refer to the appropriate OEM service manual.
Conditions for Running the Diagnostics:
This diagnostic runs continuously when the keyswitch is in the ON position or when the engine is running.
Conditions for Setting the Fault Codes:
The Engine Control Module (ECM) detected the ambient air temperature signal voltage was out of range low.
Action Taken When the Fault Code is Active:
• The ECM illuminates the amber CHECK ENGINE lamp and/or the malfunction indicator lamp (MIL) immediately when the diagnostic runs and fails.
• A default value for the ambient air temperature reading is used.
• The Idle Shutdown Ambient Air Temperature Override feature will use the intake manifold air temperature sensor value to determine idle shutdown and availability of override.
Conditions for Clearing the Fault Code:
• To validate the repair, perform a key cycle, start the engine and let it idle for 1 minute.
• The fault code status displayed by the recommended Cummins® electronic service tool or equivalent will change to INACTIVE immediately after the diagnostic runs and passes.
• The ECM will turn off the amber CHECK ENGINE lamp immediately after the diagnostic runs and passes.
• The "Reset All Faults" command in the recommended Cummins® electronic service tool or equivalent can be used to clear active and inactive faults.
Shop Talk:
Possible causes of this fault code include:
• Signal shorted to ground in the harness
• Signal shorted to return or ground in the sensor.
Read More:
SPN 632 - FMI 7 (Fault Code 259)
SPN 1043 - FMI 4 (Fault Code 284)
Blog.Teknisi
Ambient Air Temperature Sensor 1 Circuit - Voltage Below Normal or Shorted to Low Source
 |
| Ambient Air Temperature Sensor Circuit |
CODES:
Fault Code: 256
PID(P), SID(S): P171
SPN: 171
FMI: 4/4
Lamp: Amber
REASON: Low voltage detected at 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 ambient air. The engine control module (ECM) uses this temperature reading to optimize engine performance. The 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 mounting location for the ambient air temperature sensor is OEM dependent. Refer to the appropriate OEM service manual.
Conditions for Running the Diagnostics:
This diagnostic runs continuously when the keyswitch is in the ON position or when the engine is running.
Conditions for Setting the Fault Codes:
The Engine Control Module (ECM) detected the ambient air temperature signal voltage was out of range low.
Action Taken When the Fault Code is Active:
• The ECM illuminates the amber CHECK ENGINE lamp and/or the malfunction indicator lamp (MIL) immediately when the diagnostic runs and fails.
• A default value for the ambient air temperature reading is used.
• The Idle Shutdown Ambient Air Temperature Override feature will use the intake manifold air temperature sensor value to determine idle shutdown and availability of override.
Conditions for Clearing the Fault Code:
• To validate the repair, perform a key cycle, start the engine and let it idle for 1 minute.
• The fault code status displayed by the recommended Cummins® electronic service tool or equivalent will change to INACTIVE immediately after the diagnostic runs and passes.
• The ECM will turn off the amber CHECK ENGINE lamp immediately after the diagnostic runs and passes.
• The "Reset All Faults" command in the recommended Cummins® electronic service tool or equivalent can be used to clear active and inactive faults.
Shop Talk:
Possible causes of this fault code include:
• Signal shorted to ground in the harness
• Signal shorted to return or ground in the sensor.
Read More:
SPN 632 - FMI 7 (Fault Code 259)
SPN 1043 - FMI 4 (Fault Code 284)
Blog.Teknisi
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