Code P0117 on temperature sensor 1 circuit low input?

Hello, have you checked the coolant level yet? Don't open the radiator when it's hot though, do it in the morning when the vehicle is still cold. If you have no heat and coolant temperature sensor with a low input code, you are probably low on coolant.
This vehicle has 2 Engine coolant temperature sensors, one for the engine and one at the bottom of the radiator most likely for cooling fan input, but the heat in the vehicle comes from the coolant running through the heater core. If there's low coolant it's not going to run through the heater core, and the engine coolant temperature sensor cannot read air temperature, it has to be in the coolant to read its temperature and that's probably why you are getting a Low Input code.
If the coolant level happens to be full, we'll go from there, but check that first and make sure it's completely full. You don't want to replace parts that are not broken.

You can also check the 2 heater core hoses the run from the engine compartment, through the firewall and see if they are warm. The coolant runs from the cooling system into the heater core (which is in the dashboard) through the firewall and back out to the engine on the other heater core hose. They are both a small gauge hoses, but they should be right next to each other with a hose clamp on each, very easy to find. Both should be pretty hot to the touch when the engine is up to operating temperature.
The 2 diagram are the Heater core hoses, those should be hot.


https://www.2carpros.com/articles/how-to-check-engine-and-battery-pack-antifreeze-levels

Thank you so much! After I drove saw a warning mechanic signal on the dashboard. I checked coolant tank was very low, and the coolant hose was leaking coolant, and checked with p0117. I refilled coolant twice after repaired hose leaking. And when l tried to remove air in the coolant with turn on high heater level, I realized heater doesn’t work.
Now dashboard warning sign was gone, and one of two p0117 codes was gone. Now still one p0117 code exist with sensor 1 circuit low input.

So, I want to replace sensor 1. But couldn’t release the cap connector on the top of sensor.
If sensor 2 is no problem on my case?
Thanks a lot for your instruction and direction?

You may still have an air pocket in the coolant system. Here are the instructions from Honda on refilling the coolant. They want you to leave the radiator cap off for 2 cooling fan cycles. This is so the thermostat will open and allow any trapped air to escape the system. Some shops will actually use a vacuum fill system to purge any air from cooling systems. But I'll find the service info on the sensor 1 for you as well so you have it.
Diagrams 2,3,4 are for ECT Sensor 1 replacement, you're having trouble with the connector in the picture 4? Those connectors that need to be squeezed together can be difficult sometimes. You dent want to break it, I will sometimes use a very small flathead screwdriver or a small pick with a hook on the end to pry very lightly under the connector tab while squeezing it by hand, to get the tab to release. You can also try pushing in at the same time. I know they can be a pain since it's been connected like that for so many years. In the 5th picture is what I use, just a very small screwdriver or pick just help get the tab to come up, be very careful though, don't force it.

When you do get it unplugged, make sure there is no corrosion inside the connector, that can cause it to be difficult to unplug as well. If you see green or white corrosion on the pins, either on the sensor or the connector, that needs to be cleaned out with some electrical terminal spray or the connector needs to be replaced.
On coolant sensors, some fail because coolant actually comes through the sensor into the connector and that also causes corrosion to occur.

Thank you for your professional comments, instructions, detailed theoretic analysis and detailed descriptions and suggestions. Following your instruction removed air, the heater function came back. Next, I will replace ECT sensor 1.

If you clear the code does it come right back?

After replaced sensor run two drive cycle the code was gone. Thank you so much for your delicate analysis and wide range instructions with many reference documents!