2000 Pontiac Bonneville Car Will Start First Came Across Thought That

The car will not start when we first came across the problem we thought that it was just the starter, so we replaced the starter. Then we realized that the security light was not turning off so we but on a modile to by bypass the security system and that didn't work so then we replaced the ingition housing and still nothing. If we trip the relay under the hood the car will start for 3 to 5 seconds then shut down. Need help please, cant afford to take it to a garage.