Why does my tire pressure read low

That would be 'Les Schwab' ... The biggest tire dealer here in the northwest. My son (31) is not the most knowledgeable about rigs, so was just going with what the tire guy said 'You probably don't really need this ... But it might be a good idea if you .. XXXXXXX' thing.
What can I say.
When he was in college and dear ol' dad bought him a good reliable 'dork mobile' to make it through his college years, I had to beat him over the head just to check the oil. Now that he has spent some real $$ of his OWN $$, he treats it like a new-born kitten. Shocking.

 

Les Schwab....take it there, you are guaranteed to get your wheels scratched. They are in just too much of a hurry. Dont ask me how I know. Thats where the grandmas with Camrys need to go. People who treat their cars like taxicabs, not people who care about their wheels. My buddy used to be an asst. manager there, he had some horror stories. Just my opinion of course. As far as I know, but not positive, I dont think any tpms sensors have replaceable batteries. I think Les Schwab screwed the guy out of a hunski to "reset" his tpms system. And screwed him on the "battery replacements" also, unless they just replaced all 4 sensors as entire units. But why do that on a 2018 car, they should last roughly 10 years. Geez.

 

LOL!! ... OK ... Just to be fair ...
I've been going there for 25 years now .. Have always gotten pretty good tires at a 'decent' price. And they don't refuse you, if you bought tires from elsewhere, to mount/balance them. I've never had any rim-scratching probs... On 1/2 dozen rigs, but I've never had fancy rims either. (I own 6 rigs, all with steel rims, so there's that ..)
And I do like the 'Let's get it done' with LS.... I don't wanna sit there for 6 hours reading 2 year-old copies of 'Field & Stream' waiting for a new set of tires ... Or having to make an 'appointment' ....
OTOH, LS does try to soak you for every little penny they can get, and at times I think they pull 'service added costs' out of their ass.
Like on a steel rim (Non TPMS) just automatically replacing valve stems (Without asking) and charging you $10.00 each .. There's some ass-hattery going on with that, so as a customer, you gotta be on top of yer game.
Well, my son ... Non car-guy, other than he pays attention to it, now that he's paying out of his own pocket for it ... He's paying for a lesson learned, hopefully, somewhere down the line.
I can only do so much ...

 

Just cuz it's digital don't make it more accurate ...