Hello! My beloved car is experiencing some serious hick-ups.
She is 6 year old, beautiful performance et al. Love it - 2015 500L Trekking with all the extras. Fastidious about always putting mid-range fuel, an attendant made a mistake (NJ) to put in low-grade. Immediately I felt something was not right. Difficult choppy acceleration, gears going back to 3, 2 even on the highway putting a lot of pressure on engine. I've had 3 (correct) refills since and yet the issue persists. If I pull aside and restart, it somewhat fixes it for a while. The engine light has come up. A check came up with two issues: tank and the (missing) cap, which I am aware of and has to live with it and the much more serious Turbocharger issue. The additional problem, folks, is that it was checked not at an FCA service shop and they had erased the code.
Drove to a FCA shop, was told that perhaps particles of the bad fuel are still lodged in and that a drain/full clean of the tank may be necessitated. However nothing can be done if the light is not on! Car is not well, drives but being forced to do so. Thank you for your advice!

Firstly get the codes read not with a generic reader but by an Independent dealer specialising in Fiat. When were the sparking plugs last looked at? Daughter has 500L TA and I drive a turbo Volvo petrol which I have serviced for many years. If the condition of the plugs is read it will tell you a lot about the fuelling of the engine, if they are not gaped per speck and past their change interval you will have trouble and the ECU will also drive the coil packs harder (trying to compensate) and they will fail. Has the fuel filter been changed? Is there water in the petrol tank (fill up at a petrol station with low tanks and this is likely). I have stopped while fuelling my own car to help Fiat drivers who cannot replace the fuel cap and lock it once they are fuelled up. I am suspicious of the E10 petrol: the neoprene seals were not expected to cope with this - I use Super Unleaded alternately and try to encourage my daughter to do the same, prevention is better and cheaper than cure!