Ang the best way to do it is have it scanned kahit wala CEL.
Mileage paps? Hindi ba yan nabaha?
Napalinisan mo na ba air filter mo? Baka madumi ang fuel tank mo, pero kung sabi mo nga kapag mainit na engine nag ok na, baka madumi na ang engine oil mo or oil filter.
Check also your MAF sensor and throttle body lalo na kung aftermarket ang gamit mo na airfilter.