The crash, which happened about 7:20 p.m. near Quail Lake Road, occurred as the fire engine was leaving the scene of an earlier injury crash involving an overturned vehicle, the fire department said.
Three occupants of the fire engine suffered minor injuries. A fourth person was also hurt in that collision.
All lanes were briefly closed as a result of the incident, according to the California Highway Patrol. The northbound side of the interstate was subsequently reopened, while two southbound lanes remained shut down amid a traffic backup that stretched for miles.
The cause of the crash was unknown.