Select T1.car_ctl_nbr into control_number from T1 ,T2 where
T1.car_init = T2.car_init and
T1.car_nbr = T2.car_nbr ;

Update T2 set car_ctl_nbr = control_number ;

Rewrite u r trigger like this

Dont use the select stmt.

Update T2 set car_ctl_nbr = :New.car_ctl_nbr Where
car_init =:New.car_init And
car_nbr = :New.car_nbr ;