Replacing the CO sensor in the Monoxor II

The CO sensor in a Monoxor II can be replaced by the user.  Do keep in mind that when any CO sensor is replaced, it is absolutely necessary to calibrate the sensor to insure accurate readings.  To read more about calibrating a Monoxor II sensor, click here.

A special note on Monoxor II sensors

The first step is to unhook the strap, slide open the battery cover and remove the remove the batteries.

Next, remove the two phillips head screws located at the bottom of the battery compartment and the larger of the two holes below the strap.  The smaller hole is there to access the calibration screw.

Gently open the case and remove the circuit board and motor/pump assembly.

With a nut driver, remove the bolts holding the three leads to the sensor.

Then loosen and remove the three straight slot screws and bolts which hold the sensor in place.

The bolts are held in place by sockets on the rear of the pump housing as seen in the photo below.

When these three screws are removed, the sensor, O ring and scrubber/filter membrane should easily be removed from the pump housing.

If the scrubber/filter membrane is dry and clean, it can be reused.  If not, a replacement pack with the filter membrane and O ring are available (Part # 24-0520, cost approximately $30).

If the only the O ring is in need of replacement, check with your local HVAC/plumbing distributor, but be certain that it is exactly the same size and diameter.

Note:  When replacing the scrubber/filter membrane, place the 'rough' side of the filter facing away from the sensor.

When you get the new CO sensor, it comes in a case that looks like this:

Do not break the seal on the container until you are ready to install the sensor.

Remove the sensor from the case and loosen the two nuts holding the spring and remove (and discard) the spring from the sensor terminals.

If necessary, clean the wire leads and reattach the wires to the three terminals making certain they are attached to the proper location.

	The RED wire should be attached to the "R" terminal.
	The BLUE wire should be attached to the "S" terminal.
	The BLACK wire should be attached to the "C" terminal.

Use a nut driver to gently tighten the bolts holding the wires to the terminals.

Reinsert the straight slot screws and bolts holding the sensor to the pump housing being careful not to over tighten.  Remember, you are just tightening them enough so that a good seal is obtained around the O ring.

When putting the fan housing back in the instrument, make sure the lip of the housing slides in the grooves in both sides of the case.

Once the Monoxor II is reassembled you absolutely, positively must calibrate the sensor..  To read more about calibrating a Monoxor II sensor, click here.