If what they discovered and told you is the correct explaination for this behaviour, no, the car isn't defective - the sensors were and they are fixed.
In which case, all is well. You still have 3 years warranty should further problems arise.
Look at it from their perspective - why should they replace the entire engine, let alone car, for a sensor fault that has been rectified, and in 24 hours? And how will they feel about you and treat you in future if you start demanding things they have no ability or obligation to offer?
I think though that if you feel that the problem is more serious than they have intimated, you could always seek the opinion of an independent motor engineer.
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99 BMW Baby Racer - red; 05 Renaultsport Clio 182 Cup - French Racing Blue - Konis - Eibachs - Remus - Turinis