Dash board lights starting flashing, then went off. emergency light flashing, but could still manually turn them on..keys removed, and lights in dash board still tried to flash, car was still running fine

Resetting the Passlock system should fix it. If it doesn't you need an auto electrician or an auto locksmith.

https://www.google.com/search?q=2001+gmc+yukon+security+system+reset

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There are separate flashers for directional and emergency lights so they might not flash at the same rate.

anti theif problem. try un hooking battery for a few to reset computer

a lot of broblem apper electric ...it start with the side window them the clock and radio stop and now the fan thow get warm
air

the manual covers this, the IMMO is active. ?
car will not crank, is that it? it must crank to start. so cranking is 1st.
you turn the key and the IMMO LAMP must NOT be showing its blocking starts. is it
do you have full batteryvoltage. 12.6v at the battery terminals
I waited a few minutes and the dash board lights started coming
that means the battery is dead.
the dash waring lights must come on ,at key on, fast
if not we fix that first. that means power lost, 12vdc power loss.
use a voltmeter. at battery

check that the brake is full off and that the switch is correctly adjusted.

the battery says 12 volts but may be dropping to 4 or 5 under a load...i would suggest jumpstarting the car and see what happens.the battery should read in the high 12 volts and not less than 9.5 under cranking. the dash lights go out on your car when starting,and the alarm will go off ,and disable the ignition if the voltage is too low in cranking the car because the computer will shut it down and activate the anti theft.....check your battery posts are tight. best of luck

It could be that chip inside key just went bad. Do u have a 3rd key? That works? If so take the 2 good keys and turn the igntion to run postion for 3 secs each then do the same for the 3rd key and this will reprogram it if the chip is ok. If you do not have 2 good keys then u will have to have a scan tool that will link up with PATS system and perform security access to reprogram it.

replace flasher, or switch with emergency flasher to test.. Should be in with fuses or in that area.

If the security light is flashing when that happens, you need to turn the key to the ON position, and let it sit for 10 minutes, so it can reset the security code. Start it up as normal and let it idle for 5 minutes.