Cummins ISX11.9 code 256 is the Cummins diagnostic trouble code covering ambient air temperature sensor 1 circuit – voltage below normal or shorted to low source on ISX11.9 and ISX12 CM2250 engines. This DiagTechPro reference summarises what the fault code 256 troubleshooting document contains, so a technician can confirm the problem before opening the full procedure and starting a repair.
What Cummins ISX11.9 code 256 means
Fault code 256 is logged by the engine control module (ECM) and reported as SPN 171 with FMI 4 (PID P171). Cummins records describe the condition as: Ambient Air Temperature Sensor 1 Circuit – Voltage Below Normal or Shorted to Low Source. When it sets, the ECM commands the amber warning lamp so the driver and workshop are alerted to the issue.
Fault code details
| Fault code | 256 |
|---|---|
| SPN | 171 |
| FMI | 4 |
| PID | P171 |
| Warning lamp | Amber |
| Applies to | ISX11.9 CM2250 and ISX12 CM2250 (2010-12) |
Circuit and component location
The mounting location for the ambient air temperature sensor is OEM dependent. Knowing where the part sits matters, because many false readings on this circuit come from harness chafing, corroded connector pins, or moisture in the OEM wiring rather than a failed component.
Diagnostic and repair overview
The document follows the standard Cummins flow: verify the complaint, read active and inactive codes, then work through the circuit checks in order. Typical steps include:
- Confirm fault code 256 is active with the Cummins INSITE electronic service tool.
- Inspect the ambient air temperature sensor circuit, its connectors, and the OEM harness for damage.
- Measure signal, supply, and return voltages against the published specifications.
- Repair or replace the faulty part, then run the engine to validate the repair.
For manufacturer background on the engine platform and its service network, see Cummins official support. You can also browse the full DiagTechPro manuals library for more ISX11.9 CM2250 material.
About this Cummins ISX11.9 code 256 manual
This page is based on the Cummins ISX11.9 and ISX12 CM2250 fault code troubleshooting information used with the Cummins INSITE service tool. Always match SPN 171 and FMI 4 against the vehicle before condemning parts, and follow the printed wiring and torque specifications exactly.
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