Roadworthiness of test car.

In a bit of a pickle. I decided to save some of my limited money by opting to use my own car for the test rather than renting an instructor's car. It had 2 warning lights on the instrument cluster, one for a lightbulb and the other for the engine light.

The lightbulb one goes away when the full beams are on, and the other one was for a faulty fuel temperature sensor. Brought the car to the mechanic's on thursday, the replacement sensor was fitted on Friday evening, the fault was cleared and the warning light was extinguished for the rest of the day.

I started the car today and the warning light is back for the same problem. The mechanics were closed today (Sunday), and I doubt they'll have time to exhaustively troubleshoot what's more than likely going to need a lot of wiring checked it if I bring it in to them tomorrow. Similarly, I sincerely doubt any ADI at the centre is going to have a time slot to rent out their car at such short notice.

If I turn up with a warning light on is the instructor going to outright cancel the test? Will they let it slide if I show them the mechanics thought it was a solved problem? If it is cancelled, will I lose my fee? Is all of this at the instructor's discretion or is it a hard and fast rule?