F-1 Engine Test Fire or F-1 Engine Test Fire activator is a Field Order Reward that appears in Onslaught's DLC map, Ignition, received via Care Package. When activated, the engine in the middle of the map will ignite and begin burning its exhaust. Players directly underneath it, or in the underground tunnels, will be incinerated.
When a player gets it, a blue marker (where the F-1 Engine is) will appear on the minimap; the marker will blink red if there are enemies in the tunnels underneath the exhaust.
Notably, the F-1 Engine Test Fire is the first map-specific Field Order Reward that can be obtained multiple times in a single match. It also seems to count as an environmental kill, therefore it counts towards the player's current killstreak.