I've had two of them, had no problems. The sensors do go bad eventually on any system. It also will alert on other hydrocarbons, I had a can of paint leak once that set mine off. Gasoline and other flammables will too.
If its old it probably needs a new sensor head.
They list their MS 2 website for $99.02, don't know if it's the same as older models.
They're located right here in Grand Rapids, Mi
Fireboy-xintex.com
866-350-9500
I have no affiliation with the company, just a satisfied customer.