SPN 4094
STOP
NOx limits exceeded — derate
The truck's control module (ECU) reports a fault on: NOx limits exceeded — derate.
SPN 4094 means a truck control module (ECU) flagged a fault on NOx limits exceeded — derate, part of the SCR / DEF system on heavy-duty diesel trucks. DERATE rates it STOP: park safely and do not keep driving until it is checked. Part usually replaced: NOx sensor (outlet / tailpipe), $280–$800 (aftermarket to dealer) plus 0.5–1.5 h labor.
SCR / DEF
Can I keep driving?
No. Stop: park the truck in a safe place and do not keep driving until it is checked.
Part usually replaced
NOx sensor (outlet / tailpipe)
$280–$800 (aftermarket to dealer)
Labor: 0.5–1.5 h
USD, part only. Labor is billed at the shop's hourly rate.
Data: DERATE verified base, updated October 2026
Related codes
Read it off your own dash
Take one photo and DERATE tells you whether you can keep driving.
Ask DERATE
DERATE assists. It does not replace inspection by a qualified technician.