Trane not fully defrosting?????

[Windseeker]

HI Everyone.

This one has me perplexed. My Brother in Law's home has a Trane Heat Pump with the Defrost on Demand feature. One cold day while helping him in the yard, I was noticing that the unit did not seem to go into defrost. I pulled off the cover and found the coils iced. I went through the checkout procedure sheet and determined that control board must be bad. The temp sensors appear to be reading ok. I replaced the board and the sensors (just to be sure) and this is still what it looks like. I can force a defrost no problem.

I have to assume that it is working to some degree, otherwise, I should have seen ice coming out of the vents.

My theories:
The coils are not clean enough and insulating the heat. It just turns off when the bottom coil is happy. I can see through the coild, but they may have a thin layer of dirt. Could that be enough?

Low on refrigerant? Perhaps it is not getting hot enough and timing out before it can fully defrost. I'm a big believer that connecting gauges should be a last resort.

- OR -

This is how it works and I should never expect the coils to be totally clear. Not sure.

What else should I troubleshoot?

It is a model TWX030C100A if that helps.

Thanks!

Windseeker