The CO / CO₂ ratio, is the ratio of measured CO divivded by CO₂.

It gives an indication of the following:

• How good a gas sample the instrument is reading
• How clean the boiler is running

For example: A new or clean domestic boiler will display a ratio of less than 0.004, a unit in need of cleaning 0.004 - 0.008 and a unit in need of major overhaul will show greater than 0.008.

This only shows a reading when a combustion test is being carried out.

"0.0000" is displayed while in fresh air.