Britax Decathlon

You should look in your area for a car seat check station. Your 4 year old probably doesn't even need the 5 point harness. My daughter has been in the seatbelt only since she is about 3-1/2. She is 4-1/2 now and only has a booster seat. I was nervous when the trooper first told us we didn't need the 5 point harness but depending ont he size of the child sometimes it can do more harm than good. We had them checked at the Monmouth rest area on the GSP. They also have a place in Brick where they check seats and sometimes in the Wal-Mart parking lots. Anyway - good luck to you

Did you know that car seats have an "expiration date"? After a certain amount of time the plastic starts to break down & weakens the safety of the seat. I would check the seats date.
I also have the Marathon & love it.

I don't have a Britax carseat, but the ones I have used always seem to have problems with the latches. What I have done is to spray the interior latch mechanism with WD40 which loosens it up. I live in New York and the latches probably get a bit rusty from our moist weather.

I have the Britax Explora Ultra carseat and although it seems quite comfortable with the extra padding, I also have problems with the buckle and it's release button. More times than not, the buckle does not click when secured, which makes me think it's not secure. So I open and re-do it several times until I hear a click sound. The instruction booklet even mentions this problem and says to clean out the system with warm soapy water, which I did but it didn't work. The strap tightener works o.k. on mine (with effort, like you said). I never bothered to contact customer service, for some reason I've chosen to stay frustrated! Let us know what happens if you do contact them.