"Unknown" Modbus Sensor state for "0" Modbus registers value

[Al-Dek]

The problem

After updating to 2025.10.1, Modbus sensors display the "Unknown" status when reading the "0" register value.
An example of a power measurement sensor is below.

#================================#
#======= MODBUS HUB PE11-H ==========#
modbus:
  - name: PE11-H
    type: tcp
    host: 192.168.2.38  # power meter
    port: 502
    delay: 0
#------------------------------------------------------#
...
#=== Slave 3.   2-d Floor Heaters&Light (HL)=====#
...
#===== 2-nd Floor HL Active Power (W) =======#
       - name: em3_2floor_hl_w
         unique_id: em3_2floor_hl_w
         slave: 3
         address: 12
         scan_interval: 5      
         input_type: input
         data_type: float32
         device_class: power
         unit_of_measurement: W
         nan_value: 0x0000
         precision: 2
#=================================#

If the power consumption is “0”(register value =0), the sensor displays the “Unknown” status.
If any load is turned on (read register > 0), the sensor shows the correct value.
CAS Modbus Scanner reads the registers without any errors - if the power is "0", the value is 0.000000, if 4,9W, the value is 4.900000.

All the “non-zero” sensors (> 30) work fine.
Everything worked fine for years before the update.
I did not find any errors in the logs.
Cleared the cache, reloaded HA, rebooted host - nothing helped.

What version of Home Assistant Core has the issue?

2025.10.1

What was the last working version of Home Assistant Core?

2025.9.4

What type of installation are you running?

Home Assistant OS

Integration causing the issue

Modbus

Link to integration documentation on our website

https://www.home-assistant.io/integrations/modbus/

Diagnostics information

logs:
homeassistant.components.modbus: debug
pymodbus: debug

2025-10-06_DEBUG modbus[partial].txt

Example YAML snippet

Anything in the logs that might be useful for us?

Additional information

No response

[home-assistant]

modbus documentation
modbus source

[Al-Dek]

I found the reason - it’s because of the nan_value.
My sensor configuration specifies nan_value: 0x0000.
I always thought that nan_value setting gives a value to the sensor when its real value cannot be received correctly or missed.
I was wrong.
The value in nan_value determines when the sensor will become Unknown.
That is, in my case, when the register value is 0, the sensor becomes Unknow by design and in accordance with the configuration!

Honestly, I don’t understand at all why this was done such way, and what the reason for such design and where it can be applied…

[crug80]

NaN value means Not a Number and it is a control value to validate the input. If it is not a number, it is obviously unknown.
Please, refer to the documentation
If triggered, the sensor becomes unknown

Before it worked because 0 value was previously not interpreted as a real value for NaN, but after the PR #153352 its behaviour is changed.

[Al-Dek]

Yes, I saw this PR, and as I wrote earlier, I realized my mistake in interpreting nan_value.

I thought that nan_value is used specifically to handle exceptions of an erroneous result of decoding sensor data.
That is, if an error occurs as a result of decoding sensor data, the sensor returns the NaN value, which can be intercepted and replaced with the one specified in nan_value.

But I really can't understand and can't find the practical usage-purpose-case of a setting that makes the sensor Unknown when receiving a certain and correct value from the sensor!
Very odd logic: if Not-a-Number=Number then Sensor_Value_Number=Unknown ...

[crug80]

Then, if it is solved, please close the issue.