cenral heating sensor baxi 105e?

hello, ive been having trouble with the ch sensor on my baxi 105e. neon 11 flashes and the boiler doesnt even try to fire. i have tested across the sensor that i think is faulty and get 1k ohm. across a similar looking sensor i get 400 ohm. what should a healthy sensor give and if i unscrew it from its thread how much water comes out? any advice greatly appreciated. gary