NHTSA says yes. https://www.nhtsa.gov/car-seats-and-booster-seats/car-seat-u...
Same thing for bike helmets - after a serious shock, they should be replaced, even if there's not visible damage.
Same thing for bike helmets - after a serious shock, they should be replaced, even if there's not visible damage.
Also, when trying to decide whether a second-hand car seat is safe, you're multiplying the probability of a seat being damaged in an accident, by the probability that the damage will significantly reduce the safety of the seat, by the probability that the seat being sold was in an accident. I understand if a parent would rather just buy a new one and not worry about this. But I see this as being a near-zero risk.
edit: I see that some child car seats do use EPP foam, which could possibly get permanently compressed during a severe impact.
I would replace my kids seats after any accident with airbags or pretensioners firing.