Stuff happens so 2 weeks is long, do something consistent with the amount of inconvenience for the septic if they had little or nothing to do with the malfunction.

Heat tape is just one more thing in the house or malfunction or break.

It hasn't been revealed if there was a CO detector, sometimes people unhook them when they think it is malfunctioning because they don't smell anything.

Then I tell the tenant to let me know if any need batteries or replacements (chirping, malfunction etc, etc). 

So I will assume the shift key had a malfunction

They were to turn it on this week but it was malfunctioning and being repaired.

Get the 10 year battery sealed ones and replace the whole device every 10 years (or earlier if they malfunction).

If you provide extinguishers you are obligated to provide proper training on it's use and in the event of a fire, where a tenant is injured attempting to contain a fire, you can be held liable.Better pray the extinguisher does not malfunction or the tenant does not use it improperly resulting in injury or death.

Tenant agrees to allow Landlord timely access to premises to do maintenance and repair.Tenant will notify Landlord in a timely manner of any damages or problems as they arise, including damages caused by Tenant, malfunction of appliances or fixtures, and water/moisture leaks and spots.Tenant will take care of the premises, keeping them clean and clear of debris.

MAINTENANCE USE AND REPORTING: Tenant shall properly use, operate and safeguard Premises, including if applicable, any landscaping, furniture, furnishings and appliances, and all mechanical, electrical, gas and plumbing fixtures, carbon monoxide devices and smoke alarms, deck, and keep them and the Premises clean, sanitary and well ventilated....Tenant shall immediately notify Landlord, in writing, of any problem, malfunction or damage with any item including carbon monoxide devices and smoke alarms on the property.