Hi. I have a Carrier gas (propane) furnace 58MCA100-11116 series 111. It shows a code 31. The evacuation blower runs for a short time then kicks off. the burners do not ignite. To date I have: replaced the pressure switch, checked in and out lines to the pressure switch for obstruction, checked the exhaust vent tubing for obstruction, replaced the hot surface ignitor and the flame sensor. Don't know where to go next. Please help.

Your pressure switch is not closing. check for a restricted exhaust or intake vent. The pressure switch may be sticking you can blow and suck lightly on the tube going to the pressure switch to get it free and see if it helps.

The evacuation blower runs for a short time then kicks off.
How long does it run - please be more specific. Does the code occur 1) while the inducer is running, or 2) after it stops? You may have a faulty inducer motor if #2.