As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
Enroute after an attempt to adjust inflight deck temperature using alternate procedures specified in the COM, smoke was detected in the cabin. The procedure was terminated (trim air was turned on again) and the smoke dissipated. Maintenance replaced the Flight Compartment Trim Air Modulating Valve with a serviceable unit. System tests were satisfactory.