After move-in, one of my tenants began a business of purchasing used or auctioned cars, doing minor repairs and cleaning in the garage, and transporting to a used car dealership to sell. I was not aware of this activity until they called to notify me of the carbon monoxide detector alarming in the bedroom above the garage.  It's a walkup townhome unit, which is side-by-side adjacent to units on either side. 6 units in all. The fire dept arrived and could not find the source of the CO, and suspected a car had been left running in the garage without adequate ventilation since the CO reading was highest there, in the bedroom above and the adjacent units garage. The tenant denied leaving a car running. I've since installed a fire/CO detector in the garage, and sent an email to the tenant urging and highlighting the association's rules that no in-home  business that causes risk to safety and health, is allowed. Also that no engines may be left running in the garage, no "major" auto repairs which causes a hazard of fire or explosion, hazardous materials that create a danger of fire or explosion are prohibited. 

My concern is a while the used/auctioned car is parked in the garage, the risk of a spontaneous fire igniting in a car engine that may have been poorly maintained (ie. leaks etc.). This would out not only the unit in peril, but the adjacent units and residents.

I'm waiting to hear from the city reagrding any codes prohibiting autorepairs.

These are very nice tenants.  But I am tempted not to renew the lease due in 2 months. Of course I can require them to obtain additional insurance liability,  but still who wants that risk to property and occupants.

Am I overreacting ? Your thoughts.