How can people committ suicide by carbon monoxide poisoning...?

There was the story 17 March this year in Russia. A couple decided to 'get intimate' in their car that was parked in their garage. Being in Russia it was pretty cold so they decided to turn the engine on so the heater could be used. Anyway they were found dead. The garage was very small and badly ventilated. The rate of suicide due to car exhausts has fallen due to the catalytic converters.