OBS Yukon Denali Refuses To Shift

[SapperDaddy]

I have a OBS Yukon Denali with 230,000 miles. I bought it as a fixer upper as I've had good luck with a OBS truck in the past. The Denali originally shifted hard into 2nd, I dealt with it and figured it would need a 4L60E swap eventually. One evening I decided to do the Seafoam treatment to the top end of the engine. The treatment caused a knock in the engine that soon after dissipated. I followed all instructions and everything seemed well. The truck sat for 2 days untill I drove to work. Upon that trip to work, the transmission began slipping and refused to go into 2nd gear. I bit the bullet and bought another 4L60E and had it installed. After installing the transmission the truck shows the exact same symptoms as prior. Still refuses to shift into 2nd and only will shift upon letting of the throttle and usually will tack at 4,000 rpms with no shift. I originally thought it may be the TPS and replaced it as well. Still no change. I also thought that maybe the knock caused by the Seafoam put the truck into limp mode. I had the codes read and reset and still no change. As of now the only code the truck is showing is P0160, a oxygen sensor code. I'm seriously at my wits ends and am seriously thinking about junking the Denali. #PleaseHelp