The injector is "fired" on command by the ECU. The ECU uses input data signals from a few sources (O2 sensor, MAP sensor, tachometer signal) to determine when, and for how long.....the injector is active. The injector will be fired to attain a specific range of O2 voltage and this is a way of leaning out, or enrichening the air/fuel mixture in response to how the outgoing exhaust is affecting the O2 sensor.
If this truck has been neglected (insofar as check light and other parameters) for a while, and now the owner wants it fixed....you could very well be looking at more than a single issue regarding how well the engine performs.
These engines/trucks are notorious for breaking wires near connectors and even chafing the wires inside the main harness against each other to the point of open circuits, or shorting between wires.
If you find any broken wires at various connectors on the engine, Toyota does provide replacement kits for individual connectors, so don't believe it if someone says you have to buy the whole harness.