What must be done before replacing fuel injectors?

Before replacing fuel injectors, it is essential to relieve fuel system pressure. This is a critical safety step to prevent fuel from spraying out when the fuel lines are disconnected. High-pressure fuel system components can be dangerous if mishandled, as pressurized fuel can cause fire hazards or cause injury to the technician. Relieving the pressure typically involves locating the fuel pump fuse or relay, removing it, and then running the engine until it stalls, which allows the pressure to decrease.

The other options, while they might be important in certain contexts, are not necessary steps for replacing fuel injectors. Draining engine oil, for example, is relevant when performing an oil change or other engine work but is not connected to fuel injector replacement. Removing battery terminals could be advisable as a safety precaution in many maintenance procedures, but it is not a specific requirement for injector replacement. Flushing the cooling system is related to maintaining engine temperature and performance, not directly applicable to injector replacement tasks.