What this code means
The vehicle speed signal is missing or implausible. What makes this code distinctive is how many systems depend on one measurement: the speedometer and odometer, the automatic transmission's shift scheduling, cruise control, power steering assist on speed-sensitive racks, stability control, and the engine module's own idle strategy all read the same signal.
So the presenting complaint is often four apparently unrelated faults at once — a dead speedometer, a gearbox that shifts harshly or refuses to lock up, cruise control that will not engage, and steering that feels wrong at speed. That combination is diagnostically valuable: several systems failing together with one common input is a much stronger signal than any of them individually.
On most vehicles the source is a sensor on the gearbox output shaft, but many modern designs derive road speed from the ABS wheel speed sensors and send it over the network instead. Establishing which applies decides everything about where to look — on a network-derived system there may be no dedicated speed sensor to replace at all, and the fault lies with a wheel sensor or the bus message.
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.