Hi, I have a Volvo v50 2007 and I started having issues with the heating system. When I press the gas the heater works properly, but if I'm in traffic, then the air coming out is cold. Garage told me its not the thermostat. Anyone know what it could be. Thanx Gerry

 

I'd suggest making sure you don't have low coolant level, or air in the cooling system. Search for instructions on bleeding / burping your car's cooling system.

It could also be a partially / nearly blocked heater core. Flushing your radiator / cooling system would help there (and is a good thing to do "just because").