The yellow light stays on. Installed a new battery. What part could be bad? It is a John Deere GT 225 with a Koler engine.

Hi,
Could be a few things. It has a regulator attached to the blower housing, part #AM106358 that could be bad or the ground wire not grounded(plastic housing so it requires a ground wire.) Could be a bad stator, part #AM33426. Or lastly the magnets under the flywheel could have come loose on some models. That would require both flywheel and stator.
The three wire plug on the regulator can be unplugged, turned 180 degrees and plugged back up to check voltages. The outer two wire are the AC going in from the stator. Should be around 26 - 36V at full throttle. The center connector is the DC and should around 14 - 16V. That should send you to the correct area.
Peace,
Clarke