Hi my 2014 Micra failed its emissions test. No lights were showing but on checking got a fault code P0037. Mechanic changed both lambada sensors (not Nissan originals) and the emissions are worse! Another mechanic says it is probably because we did not use Nissan original parts. Before I spend even more money can anyone advice please. Thanks 

The code means 'Heated Oxygen Sensor (HO2S) Heater Control Circuit Low (Bank 1, Sensor 2)' This sensor (at the back of the catalytic converter) may be faulty and is showing a low reading for the efficiency of the cat.
This could be that there is a slight exhaust blow (a bad gasket / joint) in the exhaust system and this is causing the issue

A rag held tight over the tailpipe at the rear of the car and listen for any hissing from the exhaust towards the front of the car to establish any leaks that may be present.
(obviously Health & Safety needs observing when carrying out this test)

Thank you I will pass that on 

Hi again. I took the car for a retest today after having changed spark plugs and oil and adding additive. Emissions are now fine, however, it brought up a previous fault code P03033 which was still on the system although the engine check light was not lit. It turns out that the engine check light never comes on. The centre will not pass the car until I get this sorted. Any advice please?