You need to go about it systematically to save yourself from spending on anything that does not require to be changed. Start off by taking out the plugs one by one. Check the color of the plugs. If all plugs are blackish or dark colored, the engine is running a rich mixture, but if only one or two of the plugs is dark and the others light colored, the injectors in those cylinders may be leaking. If one of the injectors are leaking, the car would take longer to start as well and would emit black smoke upon starting as well. If that is all fine and all the plugs are similar colored, next try replacing the MAF sensor. Your mechanic says that its 'working', even while its working, the maf sensor if faulty, it'll cause the mixture to become rich while the car keeps running fine. I've had the same issue a couple of years back, and replacing the MAF sensor fixed the issue. The MAF sensor brand new, costs between 3.5-4k but you can get a kabuli one from Mithu Khan ka ahata (Pindi) for around Rs. 500 which works fine after cleaning it. Another way of going about it is to try a MAF sensor out of a known properly working car just to confirm if the maf is at fault. All variants of this corolla use the same MAF sensor, 1.3, 1.6 and 1.8 altis.