Cummins ISM Code 256 is the official Cummins fault code troubleshooting PDF that documents the ambient air temperature sensor circuit on ISM CM570 (1999-02) engines. This 7-page reference from the Cummins service literature explains the circuit, the likely causes behind the yellow warning lamp, and the step-by-step checks a technician follows to confirm the repair.
Official document details
| Brand | Cummins |
|---|---|
| Model coverage | ISM CM570, ISX CM570 and Signature CM570 (1999-02) |
| System / Group | Ambient Air Temperature Sensor Circuit (DTCs) |
| Fault code | Code 256 · PID P171 · SPN 171 · FMI 4/4 |
| Warning lamp | Yellow |
| Year / Revision | Copyright 2017 |
| Pages | 7 |
| File size | 0.4 MB |
| Format | |
| Language | English |
Document table of contents
- Circuit description
- Component location
- Shop talk – possible causes
- Troubleshooting steps
- Fault code verification and clearing
Cummins ISM Code 256 troubleshooting summary
The ambient air temperature sensor is used by the electronic control module (ECM) to monitor the temperature of the ambient air. The ambient air temperature signal is used by the ECM for the idle shutdown ambient air temperature override and ICON™ features. The ambient air temperature sensor location is OEM dependent. The manual walks through 13 labelled troubleshooting steps that inspect connectors and pins, measure circuit resistance, and verify the fault code is inactive after the repair.
The resistance of the sensor varies with the temperature. The
reading you observe should compare to this table:
Temperature (°C) Temperature [°F] Resistance (ohms)
0 32 30k to 36k
25 77 9k to 11k
50 122 3k to 4k
75 167 1350 to 1500
100 212 600 to 675
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Referenced Cummins service tools and part numbers: 3824510.
Applicable truck models
This procedure applies to Cummins ISM CM570, ISX CM570 and Signature CM570 engines built in the 1999-02 range, as used in Class 8 highway trucks. Always confirm the exact engine dataplate before servicing. Full engine specifications and dealer support are available on the Cummins official website.
Who needs this Cummins ISM Code 256 manual
Heavy-duty diesel technicians, fleet service managers and independent workshop owners diagnosing a Code 256 on the ambient air temperature sensor circuit rely on this document to follow the manufacturer procedure instead of guessing at parts. Because Code 256 shares its troubleshooting logic with the ISX CM570 and Signature CM570 platforms, a single PDF supports several Cummins engine families in the same workshop.
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