This document is a Cummins ISB fault 256 troubleshooting manual for the ambient air temperature sensor 1 circuit on ISB6.7 CM2150 engines. It explains the fault reason, operating conditions, connector checks, INSITE™ service tool steps, ECM response checks, harness resistance tests, and final code clearing procedure.
Official Cummins ISB fault 256 document details
| Brand | Cummins |
|---|---|
| Model coverage | ISB6.7 CM2150 (2007-09) |
| System/Group | DTCs; Ambient Air Temperature Sensor 1 Circuit – Voltage Below Normal or Shorted to Low Source |
| Year/Revision | Last Modified: 20-Jul-2010 |
| Pages | 12 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Overview
- Circuit Description
- FAULT CODE 256
- 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 engine harness.
- Guided Step 4 – Clear the fault codes.
Applicable truck models
This file applies to Cummins ISB6.7 CM2150 (2007-09) engine diagnostics where Fault Code 256 is active or inactive. The extract identifies PID: P171, SPN: 171, FMI: 4/4, an amber lamp, and a low signal voltage condition at the ambient air temperature circuit. It also notes related references for the ISB – Barometric Pressure Sensor Circuit and ISC and ISL – Barometric Pressure Sensor Circuit, but the listed model coverage for this file is ISB6.7 CM2150. See the manufacturer information at Cummins official website.
Who needs this Cummins ISB fault 256 manual
Workshop owners, diesel technicians, and fleet maintenance staff use this Cummins ISB fault 256 manual when diagnosing an ambient air temperature sensor signal below 0.12-VDC for more than 1 second. It is useful when checking sensor connectors, ECM connector pins, signal wire shorts, and inactive code clearing with INSITE™ electronic service tool.
The procedure starts by checking whether Fault Code 256 is inactive, then inspects the ambient air temperature sensor and connector pins for loose connectors, corrosion, bent or broken pins, pushed back pins, moisture, missing seals, dirt, insulation damage, broken connector shells, and damaged locking tabs. Later steps disconnect the sensor and engine harness, confirm whether Fault Code 249 becomes active while Fault Code 256 becomes inactive, and use that response to separate a sensor issue from a harness or ECM issue.
The harness section covers ECM connector inspection and resistance tests. The document specifies checking for a pin-to-pin short circuit from the ambient air temperature sensor SIGNAL pin to all other ECM connector pins, then checking for a pin-to-ground short circuit. The listed specification is greater than 100k ohms. If a short is detected, the repair direction is to repair or replace the engine harness under Procedure 019-043.
Final steps reconnect all components, verify the fault code status after 30 seconds, start the engine and let it idle for 1 minute, and clear inactive fault codes with INSITE™. The full PDF is available in the DiagTechPro manuals library, free account required to download.
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