The way this system works is that the current draw of one plug is compared to the current drawn by the other four. When they are in balance, the light works normally. Otherwise, the light stays off during preglow and comes back on after the engine is running awhile.

You can measure the resistance of the plugs to verify, the bad one will be open. but your best bet is to replace all five plugs, since you will need to remove the intake manifold to replace them.