SPN 3516
CAUTION
DEF concentration
The truck's control module (ECU) reports a fault on: DEF concentration.
SPN 3516 means a truck control module (ECU) flagged a fault on DEF concentration, part of the SCR / DEF system on heavy-duty diesel trucks. DERATE rates it CAUTION: drive to a shop today, because the engine may cut power. Part usually replaced: DEF quality / level sensor (tank header), $350–$1,400 (aftermarket to dealer) plus 1.5–3 h labor.
SCR / DEF
Can I keep driving?
Drive to a shop today. The engine may cut power (derate) if the fault stays active.
Part usually replaced
DEF quality / level sensor (tank header)
$350–$1,400 (aftermarket to dealer)
Labor: 1.5–3 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.