Missing message from the transmission
A module stopped sending its message often enough. Usually a network or a module-power problem, not the sensor.
SPN 1231 — Missing message from the transmission
SPN 1231 is the parameter the ECM is flagging on All makes 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.