Ac blowing hot air - 2004 Saab 9-3

I assume you are talking about mercedes c300. Is that happening when you set on auto. What happens when you set temperature to lowest settings? When it blows hot, is it hot or warm? Does ac compressor works when you blow hot air? There could be few possibilities: low freon, faulty temperature sensor in the cabin, faulty ac controller, sticky ac/heater door or faulty door motor

have the ac system properly diagnosed as it indicates no gas

I think this link will help
https://www.stangnet.com/mustang-forums/threads/a-c-problem-on-95-gt.641508/

have an accredited ac shop as an ac cannot produce hot air only cold air so that means that the ac is not working (leaks low on gas etc). The hot air is from the heater in the car and the smell is probably wire overheating or the resistors are burning.

As per your problem mentioned.There are few solved help links to troubleshoot these problems.Go through the list and Click the link below:----
Air conditioner blows air but not cold air http://repairhelpcenter.blogspot.in/2012/07/air-conditioner-blows-air-but-not-cold.html

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Car overheats and Air blows hot at idle? http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

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Heater blows cold? http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html

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Heat blows cold on drivers side and hot on passenger side? http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html---------- Hot air on the driver's side vents only? http://repairhelpcenter.blogspot.com/2012/01/hot-air-on-drivers-side-vents-only.html

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2005 Chrysler PT Cruiser: A/C Blows Warm? http://whoisbyme.blogspot.in/2012/06/2005-chrysler-pt-cruiser-ac-blows-warm.html

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1992 Toyota Camry: A/C Control light Flashing and A/C not Blowing Cold? http://technoanswers.blogspot.in/2012/04/1992-toyota-camry-ac-control-light.html

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No heat, blower blows only cold? http://technoanswers.blogspot.in/2012/02/no-heatblower-blows-only-cold.html

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These will help.
Thanks.

Is the compressor running when it blows hot air ?

I would make sure the shut off valve between the cold air and warm air is engaging. this is controlled by a vacume

Go to Midas,they do great A.C.work,your Freon may have leaked out,they can test for a charge of 19.95..

If your a/c blows only hot air you need to have it charged. Hope this helps to get you cool

Check the evaporator coil by sniffing the vents,floor is best, with your tester.