Starting my Yukon xl 2004 with ignicion cylinder disconnected

SOURCE: location of fuse box/panel for 2004 yukon denali xl

There are 2 fuse panels in the Denali.
One is a panel on the drivers side, its on the dash but facing the door.

The ohter is under the dash and above the pedals, this one is much tougher to access.

Your Owner's Manual should tell you where the fuses are that you need to access.

SOURCE: 2004 yukon xl oil and fuel gauges do not work.

2 things come to mide off the bat loose or corroded ground or cluster going bad

SOURCE: the A/C not working in yukon xl 2004

there is a mode actuator just above the accelerator pedal. That could be the problem or the control head also could need to be programed.

SOURCE: the A/C not working in yukon xl 2004

If----If everything was working great just prior to replacing the battery...In a desperate attempt--just kidding.

I would disconnect the battery
touch the battery's neg & B+ cable ends together.
This discharges capacitors.
Walk away for a few minutes, reconnect the battery and test HVAC functions to see if all is well.

If not.
The GM Dealer's Scanner can recalibrate the AC/ heater Actuators.
Note - Actuators do go bad, there are several in your truck.

Good Luck

Let us know

Dave

SOURCE: 2004 Yukon XL Denali fuel gauge fluctuations

Your sending unit for the guage is part of your fuel pump in the tank. But first try your fuel pump relay, may be working intermittenly when you hit a bump or something. Or possibly a loose wire somewhere along the way. Much less expensive (worth a look).