Trane XL14i & Trane-branded VisionPro dual fuel system (4TWX4036B1000AA TUD080R9V4K4). About seven years old.
VisionPro calls for heat. Inside, the blower fan does not start. The heat pump unit fan starts and the scroll compressor (or ??) makes a humming, sometimes pulsing, sound as it tries to start.
The heat pump tries to start for about 45 seconds & then the humming stops for about 4 minutes. Then another attempt-to-start begins a cycling pattern of start failures. The VisionPro has a continuous HEAT ON. The failure may be terminated at any time by setting the VisionPro SYSTEM OFF and then back to HEAT (waiting as long as you want before setting HEAT or letting the VisionPro use its own WAIT time makes no difference).
As a new twist in the last two weeks, the problem began twice following a clunk/thump sound after the blower fan started (at which time the blower fan stopped). And once this week the humming continued for about 20 minutes before the cyclic failures started.
I have varied the time-to-OFF from 10 minutes to over an hour. In about 15 failures over the last two years, the cycle lead to a heat pump start only twice, both times after about 30 minutes. As always, setting the SYSTEM OFF clears the problem.
Two years ago, the tech replaced the XL14i defroster board and capacitor. (Boss said replace board, cap was slightly low) He is good, but has not been able to be here while the problem was actually failing since failures are unpredictable and rare (although increasingly frequent with four in the last 10 days.)
Is any test equipment available for finding any codes that may be present? Can the scroll compressor be checked/tested somehow?
Experiences, testing suggestions or WAGs welcomed.