MSA 5x use 

try again. Make sure there is actually gas in the canister. And if it still fails, contact MSA.

I tried to measure the CO in my work space using altair 5x msa gas detector but it's not showing any thing.. Co always shows Zero and oxygen shows 20.8.. How to check?

try exhaling into the pump and see if O2 level is decreasing or not

Comb means