File Read Issue

[Blade2021]

I'm having issues getting the results i'm looking for (obviously...) I am trying to read a text file, then parse it looking for a string, If it sees that string replace text on the NEXT line. This is what I have so far:

import fileinput
import sys

x = 0
value = "car"

with fileinput.input(files="testfile.txt") as file:
    for line in file:
        if x == 1:
            line = line.replace("car", "truck")
            sys.stdout.write(line)
            print("Triggered")
            x = 0
        if value in line:
            x = 1
            print("Activated")

fileinput.close()

My textfile has the following: (just as a test)

Output:
This is line 1
This is line 2
Check
Changes
apple
Apricot
grape
car
car 
truck

Am I going about this the wrong way or am I close just missing something?