Injector 1 circuit open
The ECM is commanding a device on but almost no current flows. The circuit is open somewhere: connector, wire or the coil in the device.
SPN 651 — Injector 1 circuit open
SPN 651 is the parameter the ECM is flagging on Mack 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.
The reference is free. The diagnosis is Pro.
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.