Hope you read my PM. The thing is, there are at least 2 different frequencies that the GM TPMS uses depending on model years and model vehicle.Funny, that's exactly what I found; EL 50448! Thanks!
So important to get or at least try one of the dual frequency EL 50448 . Some vendor like to pass off what they sell as "GM compatible " or some such term. And they may just be older or cheaper devices that won't work.