Diagnostic Aids
Possible causes of a Code 33 are a faulty MAP sensor, faulty wiring or terminals, or a faulty ECM. Disconnect MAP sensor and system will go to back-up mode. If misfire or idle condition remains, MAP sensor is okay.
With the ignition switch in the "ON" position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparison of the readings from a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same +/- .4 volt. A Code 33 will also set if circuit No. 452 is open, or circuit No. 432 is shorted to voltage or to circuit No. 474.