Bryant furnace , error code 3.1 failed pressure switch . Verified drains and venting are clear of debris . All 3 pressure switched where closing . Furnace was not sending 24 volts to high pressure switch . I removed circuit board to inspect front and back , also tapping the relays . When I reinstalled board furnace operated without issue . C.0 in the exhaust was over 150 ppm , I adjusted gas pressure on high side down to 2.9 inw ( factory recommended 3.2 to 3.8 ) which brought c.o down to 70 ppm on low and high fire . Possible small blockage in secondary heat exchanger . If problem persist with pressure switch error the circuit board will be to be replace or the furnace .