Cause
Worn speedometer drive gear
The mechanical drive gear taking speed from the gearbox, worn so its teeth slip or its signal becomes irregular. It applies to designs that still use a driven sensor rather than reading a toothed wheel directly, and the gear is often a plastic part meshing with a metal one.
Wear produces a signal that is intermittent or reads low rather than absent, so the module receives a plausible speed that is wrong — and speed feeds the transmission's shift points, cruise control and the stability systems. A speedometer that reads low and flickers is the direct symptom.
Plastic gears strip in a way that is obvious once inspected, and the debris is a useful indicator of how long it has been failing. Check the gear rather than the sensor, since the sensor is the part usually replaced and is frequently fine. A gear that has stripped will have shed material into the gearbox, so it is worth checking where the fragments went.
Typically a moderate job.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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