Try a 20 minute battery disconnect in an attempt to clear any remaining fault codes.
If the lamp still illuminates after disconnection and reconnection, then there would appear to be a fault still present.
A scan would be the best form of action. Cheap code readers are available easily via the internet. The more you pay, then the more you can interrogate.
174K miles is an incredible mileage for an early model 5R55N gearbox. You certainly had your monies worth from that unit. I do not think that gearbox would have lasted to that mileage had you not had the oil changed. About 130K is about the norm for a 5R55N. Often it can be considerably less. My original gearbox lasted just 54K miles.
The golden rule with replacement gearboxes is if fitting a reconditioned unit, to have the torque converter cut open, cleaned out and rewelded and balanced before pressing a refurbished or other gearbox into action. The oil cooler and pipes may also need reconditioning. Newly refurbished gearboxes fitted to used converters and coolers which have not been cleaned out are unlikely to enjoy a long life. Particulate from a used converter or oil cooler will quickly act as an abrasive on the internal parts of any newly fitted gearbox.
Sorry I`m not more encouraging.
Mike
