Owing to concerns about climate change, California and several other states have passed laws phasing out high global warming potential refrigerants in HVAC and refrigeration systems. In the near term, the most likely replacements are A2L (mildly flammable) refrigerants. Area monitoring detectors will be required for most future residential, commercial, and industrial HVAC systems that use A2L refrigerants. These detectors must operate continuously, preferably without service for many years. The UL 60335-2-40 (2019) standard requires these detectors to respond within 10 s to a concentration of 100% of the LFL. Inexpensive detectors that meet these requirements have been slow to develop which has delayed the adoption of A2L refrigerants. A technology with good potential is based on metal-oxide semiconductors (MOS). MOS detectors from Figaro are tested here, considering their response to leaks of R-32 and R-454B. They are characterized according to sensitivity, response time, recovery time, and poisoning. The sensors do not satisfy the 10 s response time for R-32 or R-454B at the 25% LFL equivalent voltage, but succeed at the 10% LFL equivalent voltage. Their steady-state output is linear with respect to the logarithm of concentration. However, both response time and linearity are subject to possible poisoning and aging. Although the MOS sensor performance is generally acceptable, poisoning and lifetime remain a concern.