XXX, POH states top of green arc is 180 degrees c, and operations above that up to 200c for no longer than five minutes. The aircraft has full CHT and EGT on all cylinders and a flashing light on the display once a cylinder exceeds 180. The CFI stated to me that it happened so fast the light flashed grabbing his attention and he watched the temp rocket through 200 within the time it took to advise the Pilot in command as soon as the throttle was reduced to idle the engine began running rough and they declared a PAN and conducted a glide approach to back to the field, subsequently the engine stopped just after touchdown. These guys are very professional and of note was the fact that all other cylinders stayed below limits. There was no evidence that the EMS sender on that cylinder was faulty but it was replaced anyway and was discounted by the inspector. They could not find anything wrong with the cylinder that would suggest leaning out but I relayed my concerns about the intake plenum to tube design that relies on an o-ring and goop to seal.