This code 256 DTCs document is a troubleshooting procedure for the Ambient Air Temperature Sensor 1 Circuit fault, voltage below normal or shorted to low source. It explains how the ECM uses the sensor signal, when the diagnostic runs, what sets the fault, and how to validate the repair with INSITE™ electronic service tool.
Official code 256 DTCs document details
| Brand | Cummins |
|---|---|
| Model coverage | ISB6.7 CM2250 (2010-12) / DTCs; extract also lists ISB4.5, ISB6.7, ISD4.5, ISD6.7 CM2150 SN, ISL8.9 CM2150 SN, ISX15 CM2250, GX CM2250, and ISX11.9 CM2250 in associated procedures |
| System/Group | Ambient Air Temperature Sensor 1 Circuit; DTCs |
| Publication no. | Not shown; associated bulletin numbers include 4022188, 4022190, 4022250, and 2883445 |
| Year/Revision | Unknown; last modified dates shown as 11-Jun-2010 and 03-Jan-2011 |
| Pages | 12 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Overview
- Circuit Description
- Component Location
- Conditions for Running the Diagnostics
- Conditions for Setting the Fault Codes
- Action Taken When the Fault Code is Active
- Conditions for Clearing the Fault Code
- Shop Talk
- Associated Procedures
- Troubleshooting Steps
- Guided Step 1 – Check the fault codes
- Guided Step 2 – Check the ambient air temperature sensor and circuit
- Guided Step 3 – Check the ECM and OEM harness
- Guided Step 4 – Check ECM calibration and clear fault codes
Applicable truck models
The document applies to Cummins electronic engine platforms listed in the extract, including ISB4.5, ISB6.7, ISD4.5, ISD6.7 CM2150 SN, ISL8.9 CM2150 SN, ISX15 CM2250, GX CM2250, and ISX11.9 CM2250. The ambient air temperature sensor mounting location is OEM dependent, so the procedure directs the technician to the appropriate OEM service manual for component location. For manufacturer information, visit Cummins official website.
Who needs this code 256 DTCs manual
This manual is for diesel mechanics, fleet technicians, and workshop owners diagnosing a low-voltage ambient air temperature sensor signal, possible short to ground, short to return, connector damage, OEM harness fault, or ECM calibration issue. It is also useful when INSITE™ shows Fault Code 249 during circuit-response checks or when repeated connector inspections and resistance tests are required.
The full code 256 DTCs PDF is available in the DiagTechPro manuals library, free account required to download.
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