Dear Liz: Our son graduated from college last year and was recently hired as a permanent employee for a company he was contracted with for the past year. He wants to buy a new car but has limited credit history.

He has a credit card he has had since starting college. He uses it lightly and pays the balance off every month. If we are asked to cosign a loan, will paying for the car positively impact his credit scores?

Answer: Yes, an auto loan if paid on time should help his credit scores, but you shouldn’t cosign for it.

Many people who cosign loans somehow miss the important point that they are putting their good credit into someone else’s hands — and that one missed payment can trash that good credit, knocking 100 points or more from their scores.

Your son may be the most responsible 20-something on the planet, but he could still make a mistake. The only time that it makes sense to cosign a loan is when you are going to make all the payments on the debt.

He shouldn’t assume that his credit history is insufficient to get a loan. He can get his FICO scores, including the auto scores most often used by lenders, for about $20 apiece at MyFico.com. He should then take those scores to his local credit union to see what interest rate he would be offered on a car loan.

If it turns out his credit isn’t quite up to snuff, the credit union may have some kind of “credit builder” personal loan that can help improve it. (Credit unions are owned by their members and tend to have better rates and terms than many other lenders.)

Since he hasn’t had an auto loan before, discuss with him how easy it is to overspend on a car when you aren’t paying cash.

The costs of insuring, maintaining and repairing a car, plus the depreciation, can be as much as the monthly payment. In other words, the vehicle is likely to cost him twice what he thinks it will.

Once he sees how much of his paycheck is eaten up by car costs, he might be willing to consider buying a used car instead of a new one or saving up to pay cash.

If he does go ahead, make sure he understands the dangers of being “upside down” on a loan. Owing more than a car is worth leaves you vulnerable if the car is stolen or totaled, since you won’t get enough from the insurer to pay off the loan.

You can buy extra coverage for the gap, but a better approach is to make a large down payment and limit the loan term to three or four years.