From Monday 4 June 2018, you will not be able to use any vehicle (not including motorbikes) for a driving test if:

its engine management light is on, or if the light does not work
its reversing lights don’t work (this only applies to cars first used from 1 September 2009). 

If the engine management light comes on during a driving test, and the car appears to be working normally, the test will continue. The driving examiner will tell you this happened at the end of the test.