Trane XR90 trouble codes

[sambronstein]

I have a Trane XR90 air handler, model TXCO48C4HPDO with BAYLPKT210B propane kit, for a heat pump with propane backup.  Seems to work OK but there is a red light blinking inside a little round clear window.  My new house has four of these units, two on the house and one each on each garage. I need to get smart on these and make sure they are 100%   Also want to know the best way to add an ammonia absorption cooler with solar thermal power to one of these units.  Also interested in fitting variable speed blowers to the house units.  Pretty noisy when they start.  My heat bill with 60 degree setting has been about $150/month for 5000 sq. ft.so everything is probably OK, but that blinking light has me bothered.  This summer I plan to add 600 sq ft of hot air solar collectors for DHW and heat. I can make solar collectors for about $4/sq ft for materials