SPN 191 FMI 8: Transmission Output Shaft Speed - Abnormal Frequency, Pulse Width or Period

Code identity

SPN

191

FMI

8

System

transmission

Vehicle classes

truck,bus,agricultural,construction,marine,industrial

Measurement and isolation sequence

1. Capture network/signal quality and update continuity first.
2. Record waveform, frequency or message timing during the fault.
3. Rule out shared power/ground and termination effects.
4. Compare source-module and receiving-module data.

Misdiagnosis trap

Do not read an SPN/FMI combination as a failed-part label. Network messages, electrical circuits, sensors/actuators and mechanical systems can trigger the same family from different root causes. On heavy-duty equipment, derate or protection behaviour may be a secondary result.

SPN/FMI evidence chain

1. Confirm the SPN parameter and source ECU address.
2. Interpret the FMI fault mode separately.
3. Measure CAN physical layer and power supply.
4. Compare live message data with independent physical measurement.
5. If derate is present, retest under load after root-cause repair.

Related centers

Heavy trucks → · Tractors/agriculture → · Construction machinery →