I think the problem is your op-amp setup. If that is supposed to be a voltage follower, the negative input (pin 2) should be connected to the output (pin 6), not to ground. Check the output (pin 6) of the op-amp with a voltmeter during operation. Chances are the output will be at one rail or the other.
You need to take a closer look at this example you posted. The co2 sensor is a differential output. Pay close attention to how it is connected to the op-amp.http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2568&d=1210174233
Edit: And the author of that article should take a better look at the datasheet of the co2 sensor. One of those 'H' pins should be connected to the power supply, and the other to ground. That is the heater.
The heater takes a bit of current. Some co2 devices do not check constantly. They use a transistor activated by another digital pin to enable/disable the heater. The device enables the heater, waits a bit, takes a reading, then disables the heater. Then it waits a while before taking another reading. No heater current during this time.