My 16 month old is 20 lbs. I'm not sure the car seat we have is the right one. It's front facing, and it has a strap with a plastic buckle thing on the end that snaps it in place. The thing is,I always hear how the plastic thing should go across their chest, but the one on my seat looks different and covers his crotch/abdomen. I got it used so I dont have an instruction manuel. Am I using the right carseat??

What brand is it? You can look it up online and see how it is supposed to be used. There should also be a model number on the seat that you can type into your computer along with the model number of the car seat. If you're really unsure, call the manufacturers that make the car seat to guide you through it. Better to be safe than sorry!

It sounds like you have a 3-point harness. It's not as safe as a five point harness. The five point goes around each shoulder, then there is a chest clip, and each side goes around the hips and you put the two ends into the buckle. How old do you think it is? Most car seats need to be replaced after 5 or 6 years.

Also they should be rear facing until a year old. On our car seat it says they must be: 1 year old AND at least 22 lbs AND at least 22 inches(I think)
I personally wouldn't feel comfortable with a 3 point harness. I mean even adults have the shoulder strap. I wouldn't let my child ride with only a wait belt and nothing over his shoulders. Plus I wouldn't really feel comfortable with a used carseat. Do you know if it's ever been in an accident? They say if it's been in an accident, you should no longer use it.

A few months ago he was 18 lbs, but way over the height limit for his rear facing seat, now he's 20 lbs, and at 16 months there's no way I could put him in a bucket seat. It does have shoulder straps, just not a chest clip, that's what I was worried about. I think it's a costco brand, and we dont have a car so I only use it once in a while.

At 16 months and 20lbs he would be safest in a convertible, 5 point harness seat rear facing.

My son is 3 years old, 32-33lbs and rear facing in a Cosco Scenera. Also, you should check the expiration date on it as 3 point harnesses are usually a bit older.