GMC Repair: 2003 gmc dENALI, 2003 gmc denali, vacuum lines

Question
cHECK ENGION COMES ON TRANSMISSION WILL NOT SHIFT INTO 4TH GEAR
i CAN OPEN AND SHUT GAS CAP  CHECK ENGINE LIGHT MAY GO OFF TRANSMISSION SHIFTS OK sOMETIMES WHEN i START UP AND GO FORWARD THE TRANSMISSION APPEARS TO DOWN SHIFT TO A LOWER GEAR (3rd)
OCCASIONALLY THE STABLETRAC LIGHT WILL COME ON.. IT WILL GO OFF WHEN I OPEN AND SHUT GAS CAP. wHAT IS THE PROBLEM.. tRANSMISSION OR SOME SORT OF GAS/OR STABLE LOCK CONTROL...ELECTRIC?/

Answer
sounds more like a vacuum/back pressure problem. I would put a code scanner on. Autozone will do it for free and tell you what the codes are and what they mean. This will not pinpoint, but get you close. I would replace the gas cap. You should have a vented cap. Also check the vacuum lines to tank and transmission.