The demand for mobile heat pump systems increases with the growing popularity of electric vehicles. One big challenge of such systems using low pressure refrigerant is the substantial drop of heating capacity at low ambient temperature conditions, when heat is most needed. The low suction density associated with low operating pressure in the evaporator is the major reason for the capacity drop. In extremely low ambient temperature, compressor speed may need to be regulated in order to prevent suction pressure going below atmospheric pressure, hence further reducing heat pumping capability. Other factors, such as pressure drop, and refrigerant maldistribution also weakens the system. This paper presents detailed and in-depth analysis of the limiting factors on low ambient temperature operation of a mobile heat pump system. Experimental data are taken for ambient temperatures ranging from -20 to 20 ̊C. Analysis of losses are presented with the aid of a simulation model.