Separate brake-switch faults from clutch-pedal and vehicle-speed input problems when cruise control suddenly stops setting or drops out.
Cruise control is designed to fail safe. If the module thinks the brake is applied, applied intermittently, or electrically implausible, it will usually refuse to engage at all. That is why P0504, P0571, P0572, and P0573 deserve attention before you start blaming throttle hardware.
On manual-transmission vehicles, a bad clutch switch can create a nearly identical no-cruise complaint. The difference is that the brake-lamp story may stay normal while the clutch-status PID or clutch-adjustment story looks wrong. If the vehicle is automatic, the brake branch becomes much more likely by default.
A missing or irrational speed signal often affects the speedometer, transmission shift logic, ABS, or multiple modules at once. If cruise is the only obvious complaint and brake-switch codes are present, speed-signal failure drops down the list.
If cruise stopped working right after brake-light weirdness, pedal work, low-voltage trouble, or switch replacement, stay in the brake-switch branch. If cruise fails together with speedometer issues or transmission behavior changes, widen the diagnosis toward the speed-input side.
A no-cruise complaint with brake-switch codes is usually a brake-signal trust problem until proven otherwise. Prove the brake PID is sane, prove the lamps and pedal adjustment make sense, and only then widen the search to clutch inputs or vehicle-speed data.