Why my 2003 Honda Pilot won't start after replacing the ignition?

Honda only sells the ignition cylinder lock as a complete assembly which includes: the wiring, lock, security sensor...etc. Therefore the new key that operates the new cylinder must be programmed to let the computer know to shut the security system off. If your vehicle starts for a few seconds then stalls, this is quite possible what needs done. You may get away with putting the old key next to the ignition cylinder and retry the starting procedure. I do not know if the interference of the new key will allow the computer to recognise the older key. Good Luck