Dazed and Confused
Join Date: May 2012
Location: Upstate New York
Re: 2001 Merc. Sable - gremlins?
Off the wall question for part of your problem: do you have a remote car starter on it? My wife has an 04 Grand Prix GTP with an after market remote on it. It will do some strange things and, according to the installer, if the signal is too strong, it may do some strange things, i.e. ghost diagnostic (replaced a coolant sensor twice because of it) start on it's own, etc.
This may not be the cause of the brake or door light, but the issue with the lock sounds like it might be up its alley. Since many ECU's provide the output for several diagnostic lights, your problem might not be the diagnostic, but the source of the signal. I work building automation which uses alot of the same technologies as ECU's, spikes in supply voltage and erroneous grounds can also cause issues on equipment I work one, I would assume an ECU would be susceptible as well.