Product Description The dual Carbon Monoxide (CO)/Hydrogen Sulfide (H2S) sensor for the GX-3R and GX-3R Pro allows one fewer sensor slot for a standard 4-gas configuration. The GX-3R detector can measure up to 4 gases with just 3 sensors, while the GX-3R Pro can measure for up to 5 gases with only four sensors. The sensor has a 3 year warranty.