Depending on how expired it is, you actually could be fined pretty significantly for it.
But, as a previous poster stated, the reason for the expiration is the plastic fatigue. Cars heat up dramatically during the summer months, exposing the car seat to extreme temperatures. then, during the winter, temperatures drop... all of this causes plastic fatigue which causes the plastic brittle making it less effective as a protective device for the child over time. Most have expiration dates of 5 years for this reason.
Likewise, any car seat that has been through an accident needs to be tossed. The car insurance company should pay replacement value.
My rule of thumb with my kids is if I know its expired, I take it to the Babies R Us trade in event, get my 20% off coupon and replace it with a new seat. If we were in an accident and they were injured because the seat was unsafe, I'd never forgive myself for something that could have been prevented with adequate precaution.
In your shoes, I'd replace it. If you go on Amazon or Walmart.com you can get a feel for the cost - some models can be picked up for very reasonable prices. If your Grandson is over 20 months, I'd go with a Graco Nautilus as you'll be able to use it through the Booster stage once he doesn't need the 5 point harness anymore. you should be able to pick up a new one for about $140.
Hope this helps.
"Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment." (Matthew 22:37-38)