I have Nissan Dayz 2013. 
My car was not picking up, when i throttled alot of sound from engine and hardly picking up. 
The mileage was satisfactory, 16-17 in long 12-13 in city with ac. 
But with ac it was hardly running. 
Today went to my mechanic. 
Iknew the faults, i didnt asked my mechanic to drive. 
I told him to open plug, filter, injectors , cat converter, throttle body. 
The CAT was blocked. 
Rest all was in good condition ( near to new). 
After the work done, the car is running like a champ. 

Attaching pics. 

The falooda showing in pic , which came out from Cat. Please multiple it by 3. that was the total amount came out. 
Sharing the experience as i see mosty people facing the same issue. 
Regards