SPN 3226 FMI 21: outlet NOx sensor, drifted low
Outlet NOx sensor reading has drifted low. Type another SPN and FMI to check it.
Outlet NOx sensor reading has drifted low.
- Check for exhaust leaks.
- Replace the sensor if it keeps coming back.
Paste the whole line if you like: "SPN 3251 FMI 0", "3251/0" or Deere's "003251.00". Plain-words guidance; the service manual wins.
What SPN 3226 FMI 21 means
Outlet NOx sensor reading has drifted low.
FMI 21: drifted low
The reading has crept low over time. Common as a sensor ages. (SAE J1939-73: Data drifted low.)
How urgent it is
Fix it today. Fix it this shift or today. Left alone it tends to become a derate.
What to check first
- Check for exhaust leaks.
- Replace the sensor if it keeps coming back.
Plain-words guidance for a foreman, not a repair procedure. Makers add their own codes and steps; the machine's service manual wins.
Other FMIs for SPN 3226
| Code | What failed | Urgency |
|---|---|---|
| SPN 3226 FMI 2 | erratic or intermittent | Fix it today |
| SPN 3226 FMI 4 | voltage low or shorted low | Fix it today |
| SPN 3226 FMI 9 | abnormal update rate | Fix it today |
| SPN 3226 FMI 10 | abnormal rate of change | Fix it today |
| SPN 3226 FMI 13 | out of calibration | Fix it today |
| SPN 3226 FMI 20 | drifted high | Fix it today |
Next time, Dozer catches the code when it fires.
Plug a CAN logger into the machine's J1939 port, or connect the telematics you have. When a code fires, Dozer opens a repair ticket with the meaning, how urgent it is and what to check.
- The code, the machine and its hours on one ticket
- Severity sorted: keep working, plan a stop, or stop now
- The mechanic gets the checklist before they drive out
- Repeat codes on the same machine get flagged
LIVE EX-04 · SPN 3251 FMI 0 · plan a stop today Photo is an AI-generated stand-in, not a customer site.
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Questions crews ask
What does SPN 3226 FMI 21 mean?
Outlet NOx sensor reading has drifted low. FMI 21 means: The reading has crept low over time. Common as a sensor ages.
Can I keep running with SPN 3226 FMI 21?
Fix it today. Finish what's safe to finish, then fix it this shift. Ignoring it usually turns into a derate or a bigger repair. If the machine derates or the stop lamp is on, stop.
What should I check first for SPN 3226 FMI 21?
Check for exhaust leaks. Replace the sensor if it keeps coming back.
How do I clear SPN 3226 FMI 21?
Fix the cause first. Most codes go inactive by themselves once the condition is gone and the engine has run a key cycle or two. Clearing it with a service tool without a fix only hides it until it comes back.
Which machines show SPN 3226 FMI 21?
Any diesel that speaks SAE J1939: excavators, dozers, loaders, haul trucks, generators and on-road trucks with Cat, Cummins, Deere, Komatsu, Volvo, Kubota, Perkins and other engines. The SPN means the same thing on all of them; the fix steps can differ, so check the service manual.
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