2001 bonneville ssei; made loud screeching noise when started; now will not start but lights n gauges come on. have been through the battery and terminals, and have replaced starter...help

I have also done the control modules. Kind of wondering if it could be harmonic balancer or crankshaft pulley because of the noise....

So the starter does not spin the engine? Check for voltage on the small starter motor wire when the key is on. You can remove the serpentine belt to eliminate the accessories from the squeal source hunt.

I took the starter off and had it tested at the parts store. they said it was running but not at full power. I bought the new starter, and still nothing.

the screeching sounded more metallic than belt. it was a sound i hadn't heard a car make :)

I agree with tenspeed, take the serpentine drive belt off and try starting the vehicle, I'm guessing either your AC compressor or Generator/Alternator has seized and will not allow the vehicle to start.

Yay! Thank you guys, took the belt off, all pulley were good EXCEPT the AC Compressor. So happy its not my whole engine...next step: do i need to discharge the system first? THank YOu!!

Yes, you will need to have a mechanic evacuate the system in order to have the AC compressor replaced. This isn't a repair that I suggest a do-it-yourselfer attempt as to be done properly you will need a R134a recovery machine.