Ford fiesta help

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?

It sounds like the mechanic identified a misfire in the car, but you didn’t leave the vehicle with them for further diagnostics or repairs. If you haven’t already, it might be a good idea to address the misfire, as it could lead to more significant issues if left unresolved. You might consider getting a second opinion or scheduling a follow-up appointment to ensure everything is taken care of properly.

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?

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