I try and tell folks the same thing when they loose a trans at 125K or there about. Most everyone of them are upset about it. Nothing new so be ready for it folks. I keep all ours serviced regularly and it pays dividends in the long run.The 4L80E was discontinued by GM in 2013, being replaced by a six speed unit. Finding a low miles used unit is going to be hard to do, and ANY transmission that has 125,000 miles on it is likely going to need a rebuild soon. It's a bell curve, of course, with some new cars never making it out of the dealer parking lot before the trans fails, others last a quarter million miles or more if the owner maintains it and avoids "fast and furious" type driving. But if you were to look at our invoices for first-time rebuilds the vast majority are going to be within a few thousand miles of 125,000 in either direction. If you find a working one with 140,000 or more on it, it's likely already been rebuilt. Easy way to tell is the valve body gaskets, OEM are usually unmarked while aftermarket (even if they are the OEM supplier) are festooned with their name and logos.
Not that you shouldn't call around, sometimes you get lucky, just be aware what "low miles" means in relation to trans life. And clean the cooler, or better yet replace the rad or bypass it with an add-on one.
This is what we give carry-in customers to clean their cooler with, flush both directions and on the last bit of cleaner in the can, catch the solvent coming out of the cooler in a few layers of clean paper towel. If you find ANY debris, you need more flushing. We use a hot flush machine, and some coolers even after a full day of flushing are still dumping out debris. New rad time for those.
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I wouldn't hesitate at $400 for the install, the warranty alone is worth that in my book.My son was able to pickup the transmission in pieces, but had to give the guy $200 for his worthless labor. It was cheaper than getting a "new" core and wasn't worth an assault charge... so he paid it. He's taken it to a reputable shop who will have it done in a couple days. The shop can also install it for an additional $400, but with that, they flush the multiple coolers on the vehicle and add a 3 year/100K mi warranty so we are highly considering doing that.