Why has a radiator fault in our Nissan Micra resulted in the air conditioning being shut down?

My sister drives a 2008 Nissan Micra 1.2 manual and noticed smoke/vapour coming out of the dash and a substance like cooking oil on the rear carpet a few days ago. We initially thought she spilled some of her shopping. However, over the weekend the car overheated twice and on calling the rescue service, they found no coolant in the radiator. On a visit to the local garage to fix the problem, they closed off some unit, filled the radiator and away she went. However the car now has no air-con and she is worried that there is a major safety issue with the car. What went wrong?

Asked on 3 August 2013 by KN, Dublin

Answered by Honest John
The 'cooking oil' reads like coolant from a leak in the heater matrix that has condensed inside the cabin of the car. Basically the engine coolant is cooled by the car's radiator at the front, but is also used to heat the interior of the car via a smaller radiator called the 'heater matrix' inside the car. If they shut off the heater as a temporary repair, then the a/c would not work, so the job is not finished.
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