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sensor wires:

* red becomes yellow

* yellow becomes red

.. then let the forum discuss about the "yellow" sensor wire ... oh, which yellow one?

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This is not a datasheet ofr the sensor. This is some second source information, probably including mistakes.

__> always use the original datasheet provided by the manufacturer. (I won´t do the search for you, so I use the given informations)

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Sensor_Rx (input to sensor) = yellow <-- red <-- uC_Tx (uC input)

Sensor_Tx (output of sensor) = red --> yellow --> uC_Rx (uC output)

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Code:

* Serial1.begin(115200);       // UART connection to sensor (TX on sensor → RX1 = pin 0)

I don´t see why Rx1 should be on pin0. And at which pin is Tx1?? .. especially when you use a non_Arduino board.

Be sure to use the correct pins.

* Serial1.write(COM);          // Send 0x55 to request data

The diagram provided shows a single LOW pulse repeated not faster than 19ms.

Your "0x55" results in 5 LOW pulses within 80 us ... surely way different than the timing diagram shows.

--> I´d start with 0x00, that results in a single 80s LOW pulse.

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Use a scope to verify your uart signals. If you have no scope wire the sensor signal lines to your first UART_Rx one ofter the other.

Klaus