Might give this a try.
Went back to the dealer and they said they can't cure it !!!!!!!
That's just barefaced bullsh/t. Insist they escalate it**. Mention the words "trading standards" prominently.
**Franchised dealers, of every marque I've come across - though I have no idea about AR - operate a regime of venal stupidity. At the bottom are the grunt minimally-trained techs (and apprentices), doing grunt routine stuff, always trying to get on bonus by working above target efficiency because their wage is typically a mere £7-£10/hr of the vast hourly rate you pay. EG a 2hr job on the tariff (what you pay) that takes 1.5hr = 133% efficiency. Around 130% is the usual sort of target management demands. If the tech's manage to put through more than that (eg more than 130% of 40hr week, 52hrs) they get paid a better rate for any further hours. And the managers get a bonus too.
All of which encourages mistakes, but means that the last thing management wants is customer cars coming back with unchargeable faults and screwing up their targets for which they get paid bonus.
There will be a few techs who have more training, more brain cells, more experience. Among them will be guys with diagnostic skills. They're better paid, so management especially doesn't like to squander them on return repairs when they could be earning, even though they're the best choice for finding what's gone wrong.
In fact there is a hierarchy of expertise that allows dealers to call in expert techs from outside the branch, and even from the factory where a major production fault is suspected.
So when a dealer says they "can't" find out what the trouble is, what they mean is "we certainly can figure out what one of our techs has done to your car, but we aren't going to because we have your money already and plan on keeping our bonuses. So go away."
Your car is probably running like crap because Darren, age 19, tried to do a cambelt change at Warp 4 speed and boogered the timing. Or he forgot to do something up because the workshop manager kept shouting at him about why he hadn't finished a different car on another jobcard that was due to be collected at 3pm and why is it still taking up a bay awaiting parts even though the service advisor should never have promised that in the first place without checking parts stock first... Or he left a latex glove in the inlet... Or forgot to plug in the EGR. Or something.
I hate franchised dealers; if there is a good one anywhere, it is despite all the above - but I've never found one. You won't find the same clusterf$cked beancounter-spreadsheet-driven corporate idiocy at good small family run and independent garages, they depend on competence, transparency and customer loyalty. And they cost less. There are plenty of bad ones, but they're easier to spot and avoid by reputation. With franchises, the apparatus of customer care, PR and chrome plated window dressing bestows a sort of awe. They seem nearer to God. They wouldn't, couldn't possibly not live up to their authority and charge that much, surely? That is just what they want you to think.