I'm sure the forum can help, we need to know a few details.
The history of your car.
Why did your car not have the sensors in the exhaust?
Please advise details of your car, model year, engine capacity.
Please let us have a few details about the sensors you installed, new or used, part numbers, price and source.
Have you installed the sensors in the correct positions, they are not interchangeable, they are quite different, one measures flow , the other measures exhaust gas composition. did you remember to remove the protection covers from the sensors before installing them?
The MoT test produces a paper printout giving the results of the emissions test, do you have the printout, please post the figures on the printout, it may give the forum members a few clues as to the problem and a solution.
Diagnose Dan for a Youtube video on sensors and how to tell if the sensors are working or faulty