I think this is the book Hosts, which is the fifth book of <PERSON> Repairman Jack series.
The possessed girl is <PERSON>'s sister <PERSON>, and the possession is by a virus called Unity that has a kind of hive mind. As you say she is temporarily freed when she uses the microwave though it's <PERSON>'s friend <PERSON> who is first possessed:
<PERSON> didn’t look at her as she placed her cup inside the microwave and started jabbing the buttons. “What’s wrong—couldn’t see enough through the keyhole?”
That stung. “Darn it, <PERSON>, that’s not fair! I’m not snooping on you!”
<PERSON> turned toward her with a sneer twisting her lips, but then her whole expression changed, flashing from smugness to wide-eyed terror.
“<PERSON>, oh please, <PERSON>, help me!” she cried, staggering forward against the counter and gripping it with white-knuckled intensity.
And as you say it's because the microwave door is cracked and leaking microwaves:
He set the oven for five minutes and started it, then ran the little tester along the edges of the door. The indicator started flashing red immediately and went into high gear when he reached the lower right corner with the cracked glass.
“That confirms it. Leaky oven.”
But at the end <PERSON> constructs a bomb to destroy the virus. 573
There is a propane tank explosion but this is caused by the bomb:
She froze her emotions as she picked up the alarm clock. Its two detonator caps dangled on their crudely anastomosed wires against her thighs as she set the timer for 10 P.M. The time was a guess, but an educated one. She’d gleaned enough from the Unity to know that its new meeting place was not close by, and that the mutation to an airborne strain would not be a few minutes’ work. She assumed—prayed—she’d be in their midst by then.
She carefully reinserted the detonators into the holes they’d previously occupied, then emptied her shoulder bag and gently settled the assembly into its bottom. A dishcloth from the sink covered the bomb, then the rest of her stuff went back in on top. 406