SPN 3361
CAUTION
DEF dosing unit
The truck's control module (ECU) reports a fault on: DEF dosing unit.
SPN 3361 means a truck control module (ECU) flagged a fault on DEF dosing unit, 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 dosing unit (pump / supply module), $700–$2,800 (aftermarket to dealer) plus 2–4 h labor.
SCR / DEF
Can I keep driving?
Drive to a shop today. The engine may cut power (derate) if the fault stays active.
Most likely causes
Crystallized dosing injector
Dosing valve coil
DEF pump pressure
Recommended tests
1. DEF pressure build test
2. Inspect injector tip for crystals
Part usually replaced
DEF dosing unit (pump / supply module)
$700–$2,800 (aftermarket to dealer)
Labor: 2–4 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.