Sometimes car doesn't start up with a immediate fast idle.

Please check the following.

Check Spark at the plugs. (all of them)
Fuel filter not clogged.
Choke working properly.
Rev the engine while shooting carb cleaner through all the ports.
pull and clean and check your mixture screws. (Remember to EXACTLY count the turns out and go back in the same). See if this helps. Mike

The 171 is an oxygen sensor code, you can look up

That alone is not the reason for stalling or idle issues

thermostat may be at fault or engine doesn't lift idle to compensate for extra load of A/C

Under the hood of your Buick, find the Air Filter canister...follow the large hose to the top of your motor...at the injection end you should see the accelerator linkage...have someone rev your motor while you are looking for the movement of this linkage...at the bottom of the cable connection you will find a small threaded insert...by turning this insert clockwise, you can see the insert is shorter...this lowers the idle...Normally this is pre-set...and many variables can cause a fast idle...if your computer is working...the idle may reset while you continue to driv...Hope this helps...

crankshaft position sensor

Look For A Vacuum Leak ,Or Check Idle Control Solinoid

check the key my sure the programme is still good

Check for vacuum leaks. I find mine with spray carbureter cleaner. Engine will speed up when leak is found. Check the hose from the cruise control to the three way tee. The black tee turns grey and falls apart resulting in a vacuum leak. Should be on drivers side firewall.

check your idle air control valve(IAC) its probably dirty and are not able to move freely
the IAC valve controls the air mixture at idol speeds

I would Look For A Vacuum Leak, Given the Fact that ALL that Work was Done in a SHORT Amount of Time, And The Brakes ARE VACUUM ASSISTED, that when Brakes are applied the ENG. Light Comes on, Seems to Me It all May be related. It does not take Much to Pop A Vacuum line OFF Accidentally