I have an o5 mazda 3s with 125000 miles. One morming i noticed there wasnt as much pep in the motor but there were no check engine lights so i blew it off. my mileage decreased from 28 mpg to 25 mpg overmight. about a year later a cel came on and it was the maf, which the dealer cleaned for me. The car ran like new again but within a hundred miles or so it was back to its power shortage and bad mileag and still no cel. i have changed the sparkplugs with ngk iradium or whatever and the car ran better, I reset the ecu and the car ran great again for about a hundred miles. any ideas?

 

125000 miles is your problem, all you can do is do a full service on it, as in change the air filter, change oil to synthetic if u havent already, make sure the plugs you got are only the right ones but are gapped correctly. If u wanna go a little bit deeper you can do a compression check, check the activity of your 02 sensors, find a place to do a 5 gas test.