A plasma injector was a component of a starship's warp drive system, the EPS, which propelled drive plasma into the warp coils. (DS9: "Captive Pursuit") Throttles regulated the flow of this plasma into the injectors. (TNG: "Booby Trap", display graphic)
If the antimatter reaction rate on an Intrepid-class vessel dropped to nine percent, the plasma injectors would lock up and the nacelles would not reinitialize. (VOY: "Resistance")
The starboard plasma injector on the Delta Flyer tended to get somewhat hot at full impulse. (VOY: "Unimatrix Zero")
In 2063, Geordi La Forge reported to Zefram Cochrane that the Phoenix's plasma injectors were online, shortly before the ship began its historic warp flight. (Star Trek: First Contact)
In 2152, Zobral complimented Commander Charles Tucker III's repairs on his damaged vessel. Tucker shrugged it off, telling him it was only a couple of clogged plasma injectors. (ENT: "Desert Crossing")
Enterprise NX-01 operated five plasma injectors. When the fifth one started to fail in 2152, Enterprise acquired one replacement and two spares from the Kreetassans after a lengthy negotiation process, although the ship could have continued normal operation with only four working injectors. (ENT: "A Night in Sickbay")
Later that year, Rianna Mayweather aligned the plasma injectors of the ECS Horizon. (ENT: "Horizon")
In 2153, Skalaar contacted Gaavrin, his brother, and told him he had captured Jonathan Archer. He planned to hand Archer over to the Klingon Empire for 9,000 darseks, which he thought was enough to get his ship, the Tezra back. Gaavrin told him that the reward wouldn't change his decision to help his brother, to which Skalaar asked him if he wanted to scrub plasma injectors for the rest of his life. (ENT: "Bounty")
In 2165 of an alternate timeline, Jonathan Archer, T'Pol, and Dr. Phlox destroyed Enterprise by overloading three plasma injectors, which sent a feedback pulse through the warp reactor to create a subspace implosion within the ship, eliminating the interspatial parasites located in Archer's brain. (ENT: "Twilight")
In 2154, an Illyrian captain offered the damaged Enterprise a few plasma injectors or some antimatter, but not a needed primary warp coil. (ENT: "Damage")
Later that year, Lorian, Charles Tucker III's son from an alternate timeline, stole all five of Enterprise's plasma injectors from the injector assembly for use on his Enterprise so he could meet with Degra in Jonathan Archer's place. Lorian did this to reduce the chance of Enterprise being thrown again to the year 2037 in the Kovaalan nebula. Earlier, Enterprise had gained upgrades from Haradin traders for the plasma injectors to allow Enterprise to travel at warp 6.9 for brief periods. Lorian's Enterprise could not reach this speed as the plasma injectors were too old. (ENT: "E²")
In 2370, Geordi La Forge was unable to shut down the plasma injectors of the Enterprise-D because Lieutenant Daniel Kwan had locked them out so he could commit suicide by jumping through the plasma stream in one of the Enterprise's nacelles. (TNG: "Eye of the Beholder")
Later that year, a plasma injector leak aboard the runabout USS Rio Grande caused the vessel to enter the Bajoran wormhole with an only partially-collapsed warp field. This had the effect of transporting the runabout, along with its two crewmembers, Major Kira Nerys and Doctor Julian Bashir, to a parallel universe. Kira and Bashir were later able to return to their own universe by recreating the accident and going back through the wormhole. (DS9: "Crossover")
In 2371, Neelix fretted about having a child with Kes while she was undergoing the elogium and warned her that their child might start playing with the USS Voyager's plasma injectors. (VOY: "Elogium")
In 2372, while running a shuttle simulation with Kes on Voyager's holodeck, Tom Paris clogged up the plasma injectors on the shuttlecraft to make the simulation more difficult. Kes referred to it as a "dirty trick". (VOY: "Parturition")
In 2373, Chief Miles O'Brien was unable to bring the plasma injectors of a crashed Jem'Hadar warship online due to the ion regulator being damaged. (DS9: "The Ship")
The same year, Tieran occupying Kes' body remodulated the plasma injector of his shuttle to suppress its warp signature. (VOY: "Warlord")
Later, the plasma injectors on Voyager were malfunctioning as a result of the plasma flow in the manifold being constricted. Neelix suggested to B'Elanna Torres that she phase lock them to the dilithium matrix, which she'd already done. (VOY: "Fair Trade")
A little later, an attack by the Nerada took the plasma injectors of the USS Voyager off-line so the ship couldn't engage warp drive. (VOY: "Favorite Son")
In 2374, a plasma injector conduit was among many items stolen from the Voyager by Tau and his pirates. (VOY: "Concerning Flight")
In 2375, The Doctor had a holographic image of Torres' foot stuck in a plasma injector. (VOY: "Nothing Human")
Later that year, Torres tried to shut down Voyager's plasma injectors and the warp core in an effort to avoid detection from the Devore Imperium in the Tehara system. It was ultimately unsuccessful. (VOY: "Counterpoint")
In 2376, a Borg cube damaged the Delta Flyer's plasma injectors so the warp drive couldn't be engaged, despite Harry Kim's attempts to clear the injectors. (VOY: "Collective")