Stiff throttle, won't return, or throttle no longer responds

⚠️ Safety first: if the throttle stays open, returns poorly, or if the engine revs won't drop when you let go of the throttle, immediately kill the engine with the kill switch (red button) and stop. A throttle stuck open is one of the most dangerous faults there is: the bike keeps accelerating even as you release the grip. This diagnosis is done with the engine off and the bike stationary, never while riding. Two families of bikes, two fault logics. On a cable bike (the majority), the grip pulls a sheathed steel cable that opens the throttle butterfly; one or two return springs (at the butterfly, sometimes also at the grip) must close it instantly as soon as you let go. Typical causes: a dry cable, seized in its sheath or badly routed (rubbing, crushed when steering), a grip badly refitted after servicing (too much grease, poor alignment, fairing pinching the cable), or, more seriously, a broken or weakened return spring that no longer closes the butterfly on its own. On a bike with an electronic throttle (ride-by-wire, no cable between the grip and the engine), a faulty grip position sensor (APS) or throttle position sensor (TPS) can send the ECU into limp mode: revs stuck at idle, no or erratic throttle response, often with the check-engine light on. You tell them apart with a simple test at a standstill (does the grip return on its own, fast and fully?), by the context (right after a service or a refitted fairing?) and by the throttle type (cable or electronic). Urgency threshold: maximal for anything to do with grip return — never ride with any doubt on this point, and have it checked before getting back on the road.