These gas detectors have been sitting in box since the time of purchase, one was opened up and sent to Industrial Scientific to have the sensors checked and these units will need the 02 sensors replaced due to their age. quote from Industrial Scientific was $1000 for 02 sensor and recertification 

Ibrid MX6 Multigas Detectors

Model MX6 – KG235211

Configuration: H2S, O2,SO2,CO/H2, Pentane

Battery: Li-Ion ER

Quantity available: 10

Price per unit:  $1500 with no recertification