The Strongest Clue Was Not a New Part — It Was the A/B Test
With the home vent connected, temperature rose and installation mode ended in Flow Sense d80. With the vent removed, the error did not appear. That comparison survived both a thermistor replacement and a control-board replacement.
Why visible airflow does not measure the whole exhaust system
Air coming from a roof cap proves that the path is not completely blocked. It does not show the pressure the blower must overcome under heat and full flow. Duct length, fittings, a shifting liner, termination hardware and outside air can change that load.
A similar symptom can have a different cause on another LG dryer. Internal lint, blower trouble, a temperature-sensing problem or an exhaust restriction can all influence Flow Sense.
What the connected-versus-disconnected test established
Connected
Installation mode produced Flow Sense and d80.
Disconnected
The reported error did not occur with the house duct removed.
Parts already tried
Replacing the thermistor and control board did not remove the vent-dependent behavior.
How to continue without guessing at another part
The next useful evidence is measured airflow or static pressure at defined points, followed by inspection of the full duct and termination under operating conditions. The blower wheel, internal housing and damper movement also need to be verified rather than inferred from sound.
The dryer is using indirect evidence such as temperature behavior to infer poor flow; the source did not identify a separate pressure sensor. That makes the controlled comparison more useful than replacing another electronic part.
What remains unresolved in this field report
No crushed section, failed blower or termination defect was confirmed before the discussion ended. The page therefore records a strong isolation test, not a final repair. The same symptom on another installation may still have a different cause.