Cummins ISM Code 221 documents the Ambient Air Pressure Sensor Circuit Low Input 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 221 (PID P108, SPN 108, 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 | 221 — PID P108, SPN 108, FMI 3/3 |
| Warning lamp | YELLOW |
| Pages | 6 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Circuit description
- Component location
- Shop talk
- Troubleshooting steps
- Fault code verification and clearing
Cummins ISM Code 221 diagnostic steps
This fault reports a low signal from the ambient air pressure sensor located below the ECM. Technicians monitor the pressure reading with a service tool to confirm it matches actual atmospheric pressure, inspect the sensor and harness connector pins, and check the signal, supply, and return circuits for shorts to ground or open circuits before repairing the engine harness or replacing the sensor and clearing the code.
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 pressure sensor. The ambient air pressure sensor is located below the ECM. 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 221 manual
Heavy-duty diesel technicians, fleet service teams, and owner-operators diagnosing a yellow-lamp ambient air pressure 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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