Car stopped running on highway and wouldn't

salesman and shop mechanic blah bl;ah to sell a vehicle
try this simple test
when stopped apply the hand brake
start the engine and if it stays running , put your foot on the brake
if it stops but restarts when you take your foot of the brake to start it , the the problem is more than likely a failing brake booster
also to check for other problems take it to a service center or autozone and have the fault codes read

Check for vacuum leaks around the intake
and all vacuum hoses the idal control. Mn

if the engine missfires this will case the check engine light to come on. a scan tool can tell you which cylinder is missfiring

Have you checked the code to see what it tells you. I can say by experaince that our cut off and then one day stopped aty a stop light. It had to have the computer reprogramed and we were told this was an issue with this car. It could be as simple as a tune up, gas pump...etc. Check the code and it can save you time and money.

you need to see what trouble code was set when the check engine light came on

Take it back to where you got your spark plugs installed and see if you have a bad plug or if it was installed wrong.

yo i also thing it could be the timing belt

Start with changing the fuel filter. If that doesn't do it, then change the fuel pump.

the 1st thing to do is go to any Kragen or Autozone parts store and have them run a trouble code test for the reason the light is on, it is free, then post that code back here.

You likely need a new fuel sending unit (it's in the trunk, and is what drives the gauge).