SPN 4094: NOx high: poor DEF quality
NOx is over the limit because the DEF is bad. Type another SPN and FMI to check it.
NOx is over the limit because the DEF is bad. This leads to a derate.
- Test the DEF with a refractometer.
- Drain and refill with fresh DEF from a sealed jug or a clean tote.
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 4094 means
NOx is over the limit because the DEF is bad. This leads to a derate.
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
- Test the DEF with a refractometer.
- Drain and refill with fresh DEF from a sealed jug or a clean tote.
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 4094
| Code | What failed | Urgency |
|---|---|---|
| SPN 4094 FMI 31 | condition exists | 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 4094 mean?
NOx is over the limit because the DEF is bad. This leads to a derate.
Can I keep running with SPN 4094?
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 4094?
Test the DEF with a refractometer. Drain and refill with fresh DEF from a sealed jug or a clean tote.
How do I clear SPN 4094?
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 4094?
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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