How do you know if a 1990 K1500 has the heavy duty chassis?

The RPO code (Regular Production Option) for the heavy duty chassis in a 1990 K1500 is NHT. To find the RPO codes for your vehicle, look for a sticker, usually located in the glove compartment or on the door, that lists all or most of the RPO codes for your vehicle. If the truck has the NHT code, then it has the heavy duty chassis.