This just happened to me too. Had my other car in for a repair and as I was
leaving, asked the mechanic if the "air door' on the heater was electrically
actuated or vacuum?
He said it was vacuum and usually when that happened, it was a broken vacuum
line running up to a vacumm accumulator ball mounted on the underside of the
hood, driver's side.
He walked me out to my Sonoma, we raided the hood and sure enough, the
vacuum hose was broken at the black plastic ball on the under side of the
hood. He replaced the short hose and now everything works great.
Hope this helps...
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