Cummins fault code 3749

What it means

Cummins' description: Aftertreatment 1 Outlet NOx Sensor - Data not Rational - Drifted High.

SPN 3226 is Aftertreatment 1 Outlet NOx. FMI 20 means the reading has slowly drifted higher than it should be over time, typical of an aging sensor.

NOx sensors sit in the exhaust before and after the SCR catalyst. They measure nitrogen oxides so the ECM can control DEF dosing and confirm emissions are under control. Each sensor has its own small control module and talks over the J1939 network.

What the lamp means

An amber (yellow) warning lamp means the engine can usually keep running, but the fault should be repaired soon, ideally the same day.

Common causes

The sensor reading itself is out of range or the condition is real, so look at the system:

• Aging NOx sensor reading wrong
• Exhaust leak near the sensor
• Real high NOx from an EGR, DEF or SCR problem
• Sensor contaminated by oil or coolant

What you can check before calling

• Look for exhaust leaks or soot near the sensor locations
• Check the sensor connectors and that the sensor modules are mounted and dry
• Note whether other SCR or DEF codes are active at the same time

Clearing the code without fixing the cause usually brings it right back, and on emissions systems it can restart derate timers. For the bigger picture, read our guide: DEF warning lights and derate: what diesel drivers need to know.

Engines and sources

Listed by Cummins for: QSB 6.7, QSL 9, QSX 15, QSB 4.5, QSF 3.8 (With SCR), QSG 12. Cummins uses the same fault code numbers across its electronic engines, including on-highway models like the ISB/B6.7, ISL/L9 and ISX/X15, though not every code applies to every engine or software level. Source: Cummins Tier 4 Fault Codes. Always confirm with Cummins service information (QuickServe) for your engine serial number.