Ford fiesta car

I have a 2015 ford fiesta 1.25, i have taken it to a mechanic and they found out there was a misfire in cylinder 4, i have had my spark plugs replaced and thought it would fix the issue. when it starts up it is very low rpms and sometimes concs out, there is also white smoke coming out of the exhaust. once is driving its like nothing is wrong and its normal. any suggestions on what is wrong would be brilliant, thanks

Any codes that popped up? Check engine light on?

The mechanic discovered a misfire, and you didn’t leave the car there for further inspection?

check engine light comes on temporarily then goes off after a minute

they put new spark plugs in which they thought would solve the issue as a misfire is caused by faulty spark plug, injectors or ignition coil. it didn’t misfire the first day i had it back, then it started again along with white smoke and i fear it may be head gasket?

I’d take it back to them, since they charged you money to fix it, but it wasn’t solved. I’m not saying they’ll continue working on the car without extra costs, but if they value their reputation, they will at least try to solve the issue at a reasonable price and with high priority