Sold a 13 Yukon XL 4wd with 200k and next day the buyer called it doens't shift when he was stuck at traffic light. It will slightly move sometimes in reverse but won't go further either in 2wd or 4wd mode. Surprised how it went bad in 1 day. Transmission line to radiator has very minor leak but the fluid level has been good, fluid color is little dark/gray but no metal traces. Was running good prior to sale. Posting here just to get views if it's 100% bad tranny or could possibly be something else. The mechanic says 'change the transmission' he didn't look it deeply though. No codes, the only code is for oil pressure sensor and vehicle makes a weird noise when starting just like a bad starter but it's not the starter since it does it after it starts and comes from engine for a little while, noise was there previously but it's louder now. Will the tranny make such noise? I was looking online this is the same tranny for 2013-14 GM with 5.3 and 6.2 models. Appreciate the views on this thread.