Cummins ISB4.5 CM2150 Fault Code: 1943 PID: SPN: 3555 FMI: 1 Ambient Air Density – Data Valid but Below Normal Operating Range – Least Severe Level.

Circuit Description
The ambient air pressure sensor responds to pressure changes in the atmospheric pressure. The pressure changes occur based on elevation that the engine is currently operating at. The ambient air pressure sensor has a 5 volt supply circuit, a sensor signal voltage, and a return circuit.

The engine control module (ECM) provides a 5 volt supply to the ambient air pressure sensor on the sensor supply 1 circuit. The ECM also provides a ground on the sensor return circuit. The ambient air pressure sensor provides a signal to the ECM on the ambient air pressure sensor signal circuit. This sensor signal voltage changes based on the pressure of the atmosphere. The ECM will detect a low signal voltage when the vehicle is operating at high altitudes. The ECM will detect a high signal voltage when the vehicle is operating at sea level altitudes.

Component Location
The barometric pressure sensor is mounted on the main branch of the engine harness near the ECM.

Conditions for Running the Diagnostics
This diagnostic runs continuously when the engine is operating.

Conditions for Setting the Fault Codes
The ECM detects that the ambient air pressure reading is lower than a calibratible value.

Action Taken When the Fault Code is Active
-The ECM logs the fault code immediately when the diagnostic runs and fails.
-Engine torque output will be limited due to the engine operating at high altitude conditions

Conditions for Clearing the Fault Code
The ECM will return full torque to the engine and the fault code will go inactive when the vehicle is being operated within the normal altitude conditions.

Shop Talk
When the engine has been operated in a high altitude location, this fault code is logged. This fault code is for information only and no repairs are necessary. The engine can possibly be derated when operating in the high altitude location to protect the turbocharger and engine from excessively high exhaust temperatures. The derate limits the torque output of the engine to keep the temperatures below the maximum temperature limits.

Note: This fault code will most likely not be active with no load in the shop. The engine must be under the same operating conditions (altitude and temperature) at the time that the fault code had originally logged.