Cummins ISM Code 249 documents the Ambient Air Temperature Sensor Circuit diagnostic and troubleshooting procedure for the Cummins ISM CM570 (1999-02) engine platform. This 6-page fault code sheet from TruckFaultCodes.com (Copyright 2017) covers fault code 249 (PID P171, SPN 171, FMI 3/3, YELLOW lamp) and the checks used to confirm and clear it.
Official document details
| Brand | Cummins |
|---|---|
| Model coverage | ISM CM570, ISX CM570, Signature CM570 (1999-02) |
| System / Group | Diagnostic Trouble Codes (DTCs) |
| Fault code | 249 — PID P171, SPN 171, FMI 3/3 |
| Warning lamp | YELLOW |
| Pages | 6 |
| File size | 0.4 MB |
| Format | |
| Language | English |
Document table of contents
- Circuit description
- Component location
- Shop talk
- Troubleshooting steps
- Fault code verification and clearing
Cummins ISM Code 249 diagnostic steps
Diagnosis starts by disconnecting the engine harness from the ambient air temperature sensor and inspecting for corroded, bent, or moisture-affected pins. Technicians compare sensor resistance against the temperature table (for example 9k to 11k ohms at 25 degrees C / 77 degrees F), then measure resistance from pin A to pin B (175 ohms to 244k ohms) to decide whether to replace the sensor or repair the harness. The signal supports the idle-shutdown ambient air temperature override and ICON features.
Applicable truck models
This sheet applies to Cummins ISM CM570, ISX CM570, and Signature CM570 (1999-02) engines built around the CM570 ECM and the ambient air temperature sensor. Sensor location is OEM-dependent, so consult the OEM troubleshooting and repair manual. The document is laid out with a circuit description, a component location note, shop-talk tips, and a step-by-step troubleshooting chart whose go-to branches route the technician through each measurement. Refer to the official Cummins engine support site for wiring diagrams and component part numbers.
Who needs this Cummins ISM Code 249 manual
Heavy-duty diesel technicians, fleet service teams, and owner-operators diagnosing a yellow-lamp ambient air temperature sensor fault will use this sheet to follow the factory decision tree, confirm the reading, and repair the harness, connector, or component instead of replacing parts on guesswork. It spells out each measurement threshold and its pass or fail branch, so the fault can be verified active, the repair validated, and the inactive code cleared with the diagnostic tool.
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