Transmission output speed erratic
The ECM is getting a signal but it jumps around or does not make sense. Nine times out of ten this is a connection, not a part.
SPN 191 — Transmission output speed erratic
SPN 191 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.
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