I had all going off last Tue in the house. Replaced the battery in one that I thought was the culprit. Came home Thur to the same scenario except for the one with the new battery wasn't going off. Replaced battery in all the remaining detectors. Each time I plugged them back into the wiring, they would go off. I'm at a loss. They are 20 years old. Is there a life expectancy for smoke detectors?

Remove the batteries. Then use some of that keyboard spray cleaner on the guts of each. Wait 30-45 minute and re-install your batteries. If you can reach them with a blow-gun from a compressor blow them off every now and then.

Thanks, I will try that. I can unplug them and take them to the garage and use my compressor.

This is a common issue on the forum recently. The sensor can go bad causing false alarms. It is recommended to replace them every 10 years. I just replaced all mine, had to re wire at each unit due to different plug configuration, but it went quick after I found the breaker for them. No issues since

check out master guard smoke detectors a little spendy but great smoke detectors

I would never trust my family's lives to a batch of 20+yr old smoke detectors.
$50 buys you four brand new ones at Home Depot.
Been there, done that last month!