Hydrocarbon doser performance
The ECM commanded a movement, the electrical side checks out, but the hardware did not move. This is a stuck or bound mechanism.
SPN 3556 — Hydrocarbon doser performance
SPN 3556 is the parameter the ECM is flagging on PACCAR engines. The FMI that comes with it tells you how it failed. Each failure mode below has its own causes and its own test order — do not treat them as the same repair.
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Every SPN and FMI page on this site stays open — no account needed. Pro members can take the code into Blue Collar AI with the VIN, symptoms and photos, keep per-truck history, and print a work order. $14.99 a month or $99.99 a year.