Deficient means not enough or not adequate. Maybe you were deficient in caffeine that day. Maybe you were lacking in study time. But no doubt about it: you came up with a deficient number of correct answers on the physics test.
If your new car fund remains deficient, you don't have enough money to buy a new car. This presents a problem if your current car is deficient in some way––say, it no longer has enough engine power to drive up a hill. You may not feel you have what it takes to get a job, but the reality is you will have to chose between indulging your own deficiencies in this area or the car's.