What does it mean?
OBD II fault code P0109 is defined as “ Manifold Absolute Pressure/Barometric Pressure Circuit Intermittent”, and is set when the PCM (Powertrain Control Module) detects a signal from the MAP (Manifold Air Pressure Sensor) that falls outside of the expected value for the reigning engine load or throttle position, or otherwise fails to correlate with the actual pressure/vacuum in the intake manifold.
All applications use data from the manifold pressure (or vacuum) to calculate an appropriate fuel delivery strategy to suit the current engine load, and to make suitable adjustments to ignition timing as well as valve/cam timing on applications where VVT or VCS systems are present. From this it should be obvious that incorrect or invalid data can cause serious disruptions in the smooth and efficient operation of an engine.
On most applications, a fault code is set when the PCM detects a mismatch between the throttle position and the signal voltage from the MAP sensor for more than four continuous seconds. The deviation from the correct sensor voltage is measured against a reference voltage for the particular application.
However, the four-second code setting parameter may differ between applications, which makes it important to consult the manual for the vehicle being worked on for accurate information on code setting parameters. Note: Regardless of the code and the system affected, the word “Intermittent” refers to the fact that there is a sporadic, unpredictable, or, well, intermittent fault in that system.
The most likely causes of codes that relate to intermittent faults include poor connections in both live and ground circuits, as well defective switches, relays, and sensors. Intermittent faults can be extremely difficult to trace and fix, since code readers generally do not specify in which part of the circuit the problem lays.
For instance, the fault could be an intermittent interruption of the reference voltage (power supply), or, as often happens, the problem could be an intermittent break in the signal voltage to the PCM (Powertrain Control Module) due to a failing sensor, wiring issues, or defective electrical connector(s).
Note that the word “intermittent” often means different things to different applications; some systems will set a code the first time an intermittent fault occurs, while others will only do so after several failure cycles. In some cases, it may be necessary to allow the fault to worsen before an accurate and definitive repair can be made.