The Cummins ISX code 3749 manual is a 8-page PDF that documents fault code 3749, the aftertreatment outlet NOx sensor data not rational, drifted high fault. It explains that the aftertreatment outlet NOx sensor is a smart device on the J1939 data link, and that this code is set when an offset in the outlet sensor reading is detected during defined exhaust-temperature and motoring conditions. The procedure is credited to TruckFaultCodes.com and covers the circuit description, component location and full troubleshooting steps.
Official document details
| Brand | Cummins |
| Model coverage | Cummins ISX11.9 CM2250 and ISX12 CM2250 (2010-12) engines |
| System / group | ISX11.9 CM2250 (2010-12) / DTCs |
| Fault code | 3749 (PID S254, SPN 3226, FMI 20) |
| Warning lamp | Amber |
| Pages | 8 |
| File size | 0.2 MB |
| Format | |
| Language | English |
Document table of contents
- Fault code 3749 description
- Circuit description and ECM operation
- Component location
- Shop talk and possible causes
- Step-by-step troubleshooting flow
Applicable truck models
This diagnostic applies to Cummins ISX11.9 CM2250 and ISX12 CM2250 (2010-12) engines. The aftertreatment outlet NOx sensor is usually located in the exhaust at the outlet of the SCR catalyst, and is serviced only with its NOx control module. The reported cause is an offset in the outlet NOx sensor reading, with no direct effect on performance. For engine data and authorised service, visit the Cummins official website.
Who needs this Cummins ISX code 3749 manual
Heavy-duty diesel technicians, fleet workshops and owner-operators reach for this manual when fault code 3749 is logged and the amber warning lamp is on. It helps an emissions technician verify outlet NOx readings and confirm whether the sensor has drifted before replacement.
Download the ISX code 3749 PDF
The complete procedure, wiring detail and step tables are available in the DiagTechPro manuals library; a free account is required to download the PDF. Keep it alongside the related ISX11.9 CM2250 fault-code sheets listed below.
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