I also told them to do some 'engine tuning'.
Well here lies the basic trouble. Friends never, never ask for tunning, these EFi cars do not require any if you are not facing any issues like missing or starting after investigating everything else.
Believe me I have an Xli and have been maintaining at 3S dealer, but always asked them not to touch the origninal tuning and after five years MashaAllah it is performing very well.
I changed spark plugs, when I went for CNG conversion in March 08 and it was the first time I used some part from open market as the guy was trusted and these plugs are performing very well after 20K kms.
Instead of buying spark plugs from 3S get them from open market from a reilable parts dealer adn follow the code mentioned in the stickers in the engine bay or the car's manual.
Also AT oil needs to be changed after 15K Kms, manual tranny oil changes 40K. I changed even power steering fluid which is basically AT fluid and break oil as well.