Q: Runs fine with the map sensor vacuum line plugged off

Hello, the map sensor is an input of engine load. When the map sensor is disconnected it will set a fault code which is emission base and check engine will come on,this cause the system to go into a default mode. It uses engine vacuum as a measurement of that load. There is also an electronic aspect - the map sensor uses a five volt reference (0-5 volts). Zero volts would represent low engine load and 5 volts would be high load. You need the correct vacuum at the sensor to operate. Things that could affect this are having a vacuum line that’s plugged to the sensor or blocked at the intake manifold. There would be several steps to take in a diagnosis process. The overall health of the engine can create false input information to the computer (ECM). If you’d like, a certified technician from YourMechanic can come to your location to diagnose the issue.