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The module governing cruise control, faulty so speed cannot be held. On most modern vehicles cruise control is a function of the engine module rather than a separate box, so establish which arrangement the vehicle uses before looking for a part that may not exist.

Its faults appear as invalid data reported by other modules, because cruise control consumes wheel speed, brake state, throttle position and, on adaptive systems, radar data. A fault in any of those inputs disables cruise control with a healthy controller, which is the commoner case by a wide margin.

Rule out the inputs in the order they fail. A misadjusted brake switch is the classic — the system disengages the moment it believes the brake is touched, so cruise control that will not engage at all is worth taking to the brake switch before anything else. A single failed wheel speed sensor does the same thing, and both are far cheaper than a module.