Engine management light is on again

Light came on a few weeks ago and took the car to the dealer and they said that the air flow sensor had gone.

Also took the car to another garage and they said that it was showing high emissions so could be air flow, cat or lambda. But start with the air flow.

Air flow has been replaced but light is now back on again. How can I narrow down if it's the cat or lambda.

How much am I looking at for a cat or a lambda.

Also can anyone tell me which lambda sensor do I need been on the net and some are 3 wire some are 4

The car is a 52 plate WRX Impreza

Regards

Steve

 

is the car decatted? as often trips the cat fault codes..

Simon

 

No decat on the car.

Only thing non standard is the backbox.