The security system is too secure. I just bought a '94 960 Sedan. Is it normal for the security system to inhibit starting unless I had locked the car with the keyfob. If I open the door after NOT locking the car, it will not allow me to start the car. I must get out, then lock and unlock. Also, sometimes when I have unlocked it and sit down, I hear a rather loud "Cur-Chunk" sound which I have learned means I must get out again and do the lock/unlock thing again. I noticed that the siren under the hood was disconnected as if its constant sounding had to be stopped by the former owner.

Could there be a sensor error that is confusing the alarm system, such as an indication that someone is sitting in the passenger-side front seat when no one is?

Sounds like there is something wrong with the security system. I would consider having it checked out unless you can work on it.
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Will I buy another Volvo??? We'll see....

One option I forgot to mention is you could disconnect the system completely if it isn’t important to you. That could be the most cost effective solution...:)
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Will I buy another Volvo??? We'll see....

i think thats aftermarket I have a 93 & the main reason for a nostart is the NSS I think switch getting clogged at the bottom of the shifterIt makes sure its in Park before the starter will turn