Failed NCT on emission

Hi,
Please help me with this issue on my MX5.
I have a mk1 NA Eunos from 1992.
It has failed emission test on the NCT, and it has failed by a lot
The lambda sensor has been changed on the car. The car does not have steady idle, in particular when egine is cold, it keeps stop running.
My mechanic refers to an Air Fuel Ratio sensor. Is there such a thing in this car, can I buy it from your site if it exists.
What else could cause the problem? Any help welcome, thanks.
Please help me with this issue on my MX5.
I have a mk1 NA Eunos from 1992.
It has failed emission test on the NCT, and it has failed by a lot
The lambda sensor has been changed on the car. The car does not have steady idle, in particular when egine is cold, it keeps stop running.
My mechanic refers to an Air Fuel Ratio sensor. Is there such a thing in this car, can I buy it from your site if it exists.
What else could cause the problem? Any help welcome, thanks.