Cabin heater blows hot air only

[dareno]

Hi, I have a problem that cabin heater blows hot air at any gauge position, like this:
- 60F - cold air
- 61F - 90F - hot air (the same "hot level")

When I turn temperature gauge to be warmer or cooler then it reacts correctly increasing or decreasing blower motor speed but the temperature just stays hot.

I did self diagnosis procedure from HA-58 and only issue I found was that in step 5 there should be '5' displayed only but I have '51' displayed. This code is not explained in the manual. Step 5 is responsible for checking temperature sensors: inside, outside, intake. It looks that inside and outside temperatures were correct, I don't know it intake temperature is good or not (it was 50F while outside was 6F and inside 60F).

I have LE version with automatic AC. The behaviour is the same in AUTO or ECON mode.

Can you guide me how to fix it?