-6C is not cold, -20c is perhaps cold, but should cause no starting problems - 30c. is perhaps borderline if all is in good condition and no heater or starting fluids are used. You should not have problems at -6c: check the obvious
1) don't try to start it until the intake heater cycles off - monitor the voltage gauge to ensure the heater is drawing current.
2) Is your oil perhaps heavier or single grade instead of 15w40 mineral oil, which is not a starting problem at those temps but you'll know the oil is thicker with the cold - the synthetics will make a difference when the weather does get cold.
3) double check those batteries and connections
4) when it does get cold the engine block heater helps starting if you can plug it in - it is also better for engine life to start in a warmer condition - leave it plugged in all night at -20c and below.
1) don't try to start it until the intake heater cycles off - monitor the voltage gauge to ensure the heater is drawing current.
2) Is your oil perhaps heavier or single grade instead of 15w40 mineral oil, which is not a starting problem at those temps but you'll know the oil is thicker with the cold - the synthetics will make a difference when the weather does get cold.
3) double check those batteries and connections
4) when it does get cold the engine block heater helps starting if you can plug it in - it is also better for engine life to start in a warmer condition - leave it plugged in all night at -20c and below.