1999 Park Avenue Electrical Problem

Hello all,
I have a 1999 Buick Park Avenue with 100,000 miles and recently experienced an electrical problem. Hoping someone here might see the connection between these symptoms. This all occurred at the same
time.
1. All interior lights stopped working, except the ashtray convenience light. Trunk light also inop. Cannot turn reading lights on with switches, and cannot turn on by turning headlamp knob,
2, The "Battery State" display indicates "????". I replaced the battery with a new one. No change in the indication. When starting the car, it first indicates 0 volts, then changes to ????.
3. The computer readout for MPG, average fuel consumption, and remaining range are off. It shows I go through 35 - 40 gallons a tank, and the average fuel mileage is very low. I still get the same gas mileage as before.
4. The rear window defroster button beeps when pressed, but does not light up and the window does not defrost.
5. The air conditioner compresses does not engage when running the A/C.
6. If I have the keys in the ignition with the door open, the alarm does not beep.
This started one night when I unlocked the doors with the remote and the interior lights did not work. When starting the car, the "Service Vehicle Soon" light illuminated and the air bag light stayed on after flashing the normal amount of times. I had the service light checked and there was no error code. The Service Vehicle light went out the same day, but all other problems remain.
I believe I went through all the fuses and they looked good.
Any ideas?
Thank you in advance, Igor
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Igor N. Nikishin wrote:

Hi Igor...
Not a mechanic, just an old retired electrical guy, but I experienced something very similar a few years back, so offer this for what little it may be worth...
Older Olds, pre the "hide the battery and use a jumper post" days. Multiple connections at the battery positive post. One of those connections had become so terribly badly corroded that it made no contact at all. Took all the connections apart, cleaned them all nice and shiny with a scrap of sandpaper, and it worked like new again.
I guess in yours you'd do the same, but take them apart at the jumper post.
Take care.
Ken
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Maybe I am jumping the gun, but...wait and see if Ian offers an opinion, but with this many seemingly unrelated problems, perhaps a service center/someone with a proper scan tool should make certain the BCM is alive/sane? A ground problem certainly does not seem out of the question...

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Given the bizarre and widespread nature of the OP's problems, I think a bad ground would be one of the first considerations I'd be looking at.
--

-Mike-
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Hi Igor. I'm also an old retired electrical guy. The first thing I'd check, not just look at/observe, are the battery cables/connections. The cables need to be checked for resistance and their connections need to be checked for a good/clean mechanical connection. If the cables check out good and all the ground connections/wiring are also check ok, then I'd suspect the computer. The computer controls all the areas you posted about and if its' power supply is faulty.......
I hope this helps some....
Dave S(Texas)
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