Be careful, if the TMPS has aluminum stems not all cores and caps are compatible with them. The wrong core or cap can cause galvanic corrosion problems.
Just what I was searching for @RobH! May be a newbie on this Prius board, but not to discussion boards and the search features available! My wife and I both have 2013's. She's got one tire that consistently looses pressure and triggers the TPMS warning light. When I check the pressure and/or connect my air hose to this one particular valve, I don't get that familiar yet quick hiss of air escaping until connected. I do with the other three. My first thought was that the valve stem might be bad. I already have the little tool to unscrew the valve stem and figured that'd be a good place to start. Wasn't until I started reading several threads here that it was pointed out that I couldn't use a brass valve stem that I keep as a spare for my Landcruiser because of the aluminum vs brass incompatibility. Link provided to Amazon and there's the rebuild 'kit' there for $5. As good a place as any to start.
I just replaced the valve stem core on a 2019 Prius. Nothing to do with the TPM. It's the same Schrader core that you had in your bicycle tire when you were 10! They're about $2 for a pack of four (that should be a lifetime supply). You can buy a tool for a few dollars, but I splurged for the fancy tool with two cores included for $11 at AutoZone.