I have a 12 year old Trane Xe1000 heat pump. Recently I noticed that the fan keeps running continuously even when the outside unit turns off. It's as if the tstat fan switch is set to On (it's not) instead of Auto. I tried a few things:

1) In the tstat (weathertron) I disconnected the fan control wire (green). Fan kept running.

2) I killed the power via the breaker. System shut down (including the fan). After re-powering the system, the fan DID properly cut off after the cooling cycle. But a day later, the problem is back

I opened the AHU access panel and according to the wiring diagram there is a fan delay relay (FDR). That could be sticking. I've also seen some other posts around the internet talking about the FDR on a Trane. What's the best way to check this relay (I'm experienced (and cautious) around electricity).
Wha other components could fail?

Thanks

Mitch

I had the same problem. The relay to the fan turn off the immediately turn back on. Just in the cooling, heating was fine. I had an old mechanical mercury switch Honeywell therostat. Changed t-stat to a new Honeywell, works fine. Be sure to label you wires before unwiring old t-stat.

I would check the A/C fan relay in the inside unit first. If you disconnect the fan wire in the thermostat and the fan still runs it is a good possibility the relay is stuck/defective.