We had something similar... Our heat pump works down to about 36 degrees and then kicks over to backup heat coils. We mainly heat with a wood stove, but sometimes we need the heat to work when it's cold and we are away.
Obviously, something went wrong with the resistance heat side. It was struggling for awhile, then it went out completely. The tech checked the coils, and they were fine. Replaced the limit switches, and it worked again. Great. But that lasted for all of one night.
The tech came back, tested everything, and then put a flashlight to the condenser/heat exchanger. He looked several times, frowned and asked me, "How long has it been since this was cleaned?
"Cleaned? Never, that I know of." (Over 12 years)
"This makes sense, now. The system looks really good overall, but from what I can see, this thing is packed with dirt."
He spent the next couple of hours with various detergents, chemicals and a garden hose, and it was shocking how much stuff was packed between every fin of that condenser, but water would still not go through it. I eventually pulled out an electric pressure washer set on low, and it helped to finally get a spot cleared out, and we expanded the clean area until the whole thing was done. It was crazy how much stuff was in there.
It was well after dark when we vacuumed up all the debris and water, blew everything dry with compressed air and then test-fired the system.
It works better than I can ever remember. It used to take a long time to bring up the temp 2 degrees, and it would run almost constantly to maintain temp on cold nights. Now, it kicks on for a couple of minutes and cuts off. It works incredibly well!
I really appreciate that tech sticking with it and actually fixing our problem. Other companies gave blanket statements that systems should be replaced if over 10 years old. The tech says they just want our money. He says we should easily get another 10 years out of it, no problem.
We have him a very good tip!