I ordered and received 2 units. One works, the other does not light up even it is completely off. The seller offered me a refund for the defective sensor and reimbursed me. I would have liked to compare CO values displayed with the 2 devices because I read on reviews that the values can be very different between 2 devices. Unfortunately I have not been able to do, which does not allow me to give an opinion on the quality or reliability of the product ...