Q: Car won't start - 2004 Volkswagen Golf

Hello. I will help you on this. It’s unclear whether you bought a new battery and a new alternator because those parts were accurately diagnosed as defective. A battery must be load tested, only after it is fully charged, in order to determine if it is defective.

An alternator must also be tested according to specific instructions in the Factory Service Manual (FSM). If these tests were not performed and a mechanic just told you to change out those parts, then you need a new mechanic. In the world of automobile technology, there is no diagnosis that refers to an AC system or a heating system that draws too much energy. The diagnosis the Mechanic gave is a bit imprecise. We can speak of electrical faults in the electrical segment of an A/C system as "drawing too much energy" but typically such will blow a fuse. At any rate, your A/C and heating systems and their energy consumption and the electrical and mechanical load they place in your engine have nothing to do with your no start condition. You may want to have the mechanic explain their findings back to you more specifically or have a second mechanic diagnose the no start for you. The mechanic can then resolve your issue by replacing only those parts that are necessary to get your car running. Given the age of your car, certain "tune-up" parts might be recommended as well such as plug wires, spark plugs, fuel filter and so forth the replacement of which will help you avoid future problems if those parts are past due for replacement.