My 2018 Ford focus has 168000 miles runs good but I want to do preventative maintenance on it when I have the money I'm thinking about the spark plugs and serpentine belt, alternator, and it will have brand new tires brakes were done last year what else should I get done? Transmission fluid change or flush , electric connections on transmission underneath the air box what else should I be doing?

 

1) Check my signature below (and if it's 2.0L, turbo or non-turbo)
2) What have you done thus far and when was that at what mileage?
3) What fuel economy both city/highway
4) High motor oil consumption (without leaks) or not
5) Are you doing maintenance yourself, what is your mechanic's skill level?
6) Which state are you in right now?

 

A good place to check for that info would be your owner's manual. If you don't have it you can find a copy on line.

A trans flush should not be considered due to the high mileage AND a flush can stir up a lot of sediment in the pan and send that into the valve body causing problems.
If not done yet, think about water pump, thermostat and hoses, any one of which can leave you stranded on the road if neglected.