SPN 3226: Aftertreatment 1 Outlet NOx Sensor
What SPN 3226 Is
SPN 3226 is Aftertreatment 1 Outlet NOx — the downstream NOx sensor after the SCR catalyst. It reports tailpipe NOx so the ECM can judge SCR conversion. It is not the DEF tank level sensor. Level is SPN 1761. Inlet NOx is SPN 3216.
The sensor is typically a smart CAN device on the aftertreatment datalink, with an internal heater. Failures look like circuit (FMI 3/4/5), missing messages (FMI 9), or rationality / drift (FMI 20/21) when the reading does not match engine-out NOx and DEF dosing.
OEM Overlays
- Detroit — GHG14/GHG17 TSBs cover SPN 3226 FMI 2, 20, and 21 as SCR outlet NOx drift. Procedure includes ATS leak check, DEF quantity test (108–132 mL), and looking for DEF crystal in the mixer before condemning the sensor.
- Cummins — FC 2771 is commonly SPN 3226 FMI 9 (outlet NOx abnormal update rate). FC 1887 maps to FMI 4 (voltage low) on many lists. INSITE Fault Information System is the authority for that serial.
- Navistar — SPN 3226 FMI 4/10 appear on N13/N9/10 vertical exhaust with water on the outlet sensor; there is a rain-shield TSB.
Common FMI Combinations
| FMI | Meaning | Start here |
|---|---|---|
| 2 | Erratic / drifted | DEF crystal in mixer, exhaust leak, connector |
| 3 / 4 | Voltage high / low | Harness, power, ground, short |
| 9 | Abnormal update rate | Private aftertreatment CAN, unplugged sensor |
| 10 | Abnormal rate of change | Wiring vs failing element |
| 13 | Out of calibration | After replacement, programming |
| 20 / 21 | Drifted high / low | Rationality — do not auto-replace |
Diagnostic Order
- Pull the full list. If 1569 or 5246 is active, 3226 may be the reason — or a companion.
- Visually check the ATS for exhaust leaks ahead of the outlet sensor. A leak dilutes NOx and creates false efficiency and drift codes.
- At operating temperature, compare inlet NOx to outlet NOx. A healthy SCR should show a large drop. Identical readings mean a dead catalyst or one sensor lying.
- Check DEF quality with a refractometer (about 32.5% urea) and dosing quantity before you buy a sensor. Detroit specifically calls this out on FMI 21.
- Unbolt the DEF doser and look for a solid crystal mass in the mixer. A parked regen after cleaning can restore NOx readings without a new sensor.
Outlet NOx sensors are $400–$800. A $2 connector cleaning or a mixer full of DEF crystal is still the more common root cause on FMI 2/20/21.
Companion Codes
Watch SPN 3216 (inlet NOx), SPN 4364 (SCR efficiency), SPN 3364 (DEF quality), and inducement SPN 5246. On a remote session we graph both NOx sensors on a Nexiq USB-Link 3 and a diagnostic laptop so you are not guessing which sensor is the liar.
Frequently Asked Questions
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