5 months is too far past expiration, IMO.
I had a seat a friend gave me when I was pregnant with Aeryn, a Peg Perego, which was due to expire like a month before she was born. It was in pristine condition. At first, I thought I'd be OK with using it. I'd even found an expired one at a yard sale for like $2 which I picked up (and recycled) so I'd have a spare cover. I figured I'd use it for a few months then move her into the Radian I already had because I was doing daycare before Aeryn was born.
Well, as my pregnancy progressed, I decided it was silly to skimp on a car seat and I decided I wanted a new one. DH and I settled on the Onboard Air. I put the Peg on a shelf in our (well climate controlled) garage. A few months after Aeryn was born, I thought maybe it would be a nice place for her to sit, so I got it down with the intention of using it as a "baby" chair in the house But I couldn't for the life of me get it out of the base! The plastic in the release mechanism had become too elastic and it wouldn't pull the latch open anymore. This seat wasn't even six years old! (Pegs expire after 5 years, or at least this one did; which was part of why I toyed with using it, since it wasn't even as old as most unexpired seats). I'd say it was about 4 or 5 months after its expiration and it already wasn't functioning correctly. It had only ever been "in service" for about 8 months total, split between two other kids, and had always been stored indoors or in a climate controlled environment.
That experience made me "respect" expiration dates a lot more. I saw for myself how plastic degrades and can cause a seat to fail. Would it have protected my child in an accident? Probably. But how can I know?
I'm not going to panic and stay home if I discover the seat I've got expired two weeks ago. I'll still use it to drive around town with my kiddo and shop for a new car seat, or while I wait for one to arrive from Amazon or something. But it's just not worth it IMO to intentionally use a seat months past its expiration.
Assuming they haven't sold out, Safety 1st has an Onboard Air in Silverleaf on their website for $76 right now. And you can get the stroller frame to go with it for an additional $20 (half off sale price with purchase of an onboard car seat).