fault code 256 DTCs is documented on this DiagTechPro library page for Cummins ISM CM871 (2007-09) engines, where it flags a Ambient Air Temperature Sensor 1 Circuit – Voltage Below Normal Or Shorted To Low Source condition (PID P171, SPN 171, FMI 4/4, Amber lamp). This 4-page PDF reproduces the manufacturer description, shop talk, component location and step-by-step troubleshooting so technicians can confirm and clear the code.
fault code 256 DTCs overview
Cummins defines this code as “AMBIENT AIR TEMPERATURE SENSOR 1 CIRCUIT – VOLTAGE BELOW NORMAL OR SHORTED TO LOW SOURCE” on the Ambient Air Temperature Sensor 1 Circuit. The ambient air temperature sensor is used by the electronic control module (ECM) to monitor ambient air temperature. This can be used to sense the temperature inside the cab of a vehicle or the air temperature outside the vehicle. The ambient air temperature signal is used by the ECM for the Idle Shutdown Ambient Air Temperature Override feature. The ambient air temperature sensor is used by the ECM to monitor the ambient air temperature. The ambient air temperature sensor is a thermistor and changes resistance based on the temperature being measured.
According to the shop talk notes, possible causes include:
- Signal shorted to ground in the harness.
- Signal shorted to return or ground in the sensor.
Official document details
| Brand | Cummins |
| Model coverage | Cummins ISM CM871 (2007-09); Cummins ISX CM871 (2007-09) |
| System / Group | Ambient Air Temperature Sensor 1 Circuit — ISM CM871 fault codes |
| Fault code | 256 (SPN 171, FMI 4/4) |
| Source | TruckFaultCodes.com Cummins fault code reference (© 2017) |
| Pages | 4 |
| File size | 0.2 MB |
| Format | |
| Language | English |
Document table of contents
- Fault code identification (PID, SPN, FMI, lamp)
- Description
- Shop Talk
- Component Location
- Troubleshooting Steps
Applicable truck models and engines
This document applies to Cummins ISM CM871 (2007-09); Cummins ISX CM871 (2007-09) equipped diesel engines that log Fault Code 256. Always confirm engine calibration and use a Cummins diagnostic tool such as INSITE to read live data before starting repairs. Full specifications are published on the Cummins official website.
Who needs this fault code 256 DTCs manual
Heavy-duty truck technicians, fleet workshops and owner-operators diagnosing an electronic control module warning on ISM CM871 engines will use this sheet to verify the fault before parts are replaced.
The full PDF is available in the DiagTechPro manuals library; a free account is required to download.
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