Is your sensor the TDS0045 ?
Where can I find more information ?
Perhaps here, http://www.dynament.com/infrared-sensor-data/infrared-gas-sensor-applications.htm
Why did you power it with 3.3V, I think they can be used with 5V.
Are you sure that it uses RS232 levels ? Or is it using serial 5V TTL levels ?
Are you sure the sensor itself contains all the logic for serial communication ?
I think the document for communication protocol is for the OEM board.
The sensor itself seems to be analog.
I hope you didn't damage the sensor or the OEM board with high voltage levels from the MAX3232.
The MAX3232 is for 3.3V and 5V, and the older MAX232 is only for 5V.
Can you make a photo of how it is connected ?