What does it mean?
The manifold absolute pressure (MAP) sensor is used to measure engine load. The powertrain control module (PCM) sends the MAP sensor a reference voltage and the sensor varies the voltage according to manifold vacuum. The PCM then uses this information to adjust ignition timing and fuel delivery. An example of a MAP sensor Vacuum and pressure are inversely proportional (when vacuum goes up, pressure goes down).
Because of this, the MAP sensor is able to measure the pressure in the intake manifold to gauge vacuum. Manifold vacuum is a good indicator of engine load. With the engine off, manifold pressure and atmospheric (barometric) pressure are the same. With the engine running, the movement of the pistons and restriction of the throttle plates create vacuum (reduce pressure). When the engine is under heavy load at wide open throttle (WOT), vacuum is nearly zero.
At this time, engine pressure and atmospheric pressure are almost the same. Depending on the elevation, barometric pressure is usually between 28 to 31 inches of mercury (Hg). By comparison, engine vacuum at sea level is between 17-22 in-Hg. This drops approximately 1 in-Hg for every 1000 ft. of elevation. On some vehicles, when the engine is first started, the PCM looks at the MAP sensor to determine barometric pressure. There are other vehicles, however, that use a dedicated BARO sensor.
Others still use a combination BMAP sensor. There are two types of MAP sensors: analog and frequency. The analog type uses a diaphragm and is connected to the intake manifold by a vacuum line. The sensor monitors the movement of the diaphragm and sends a corresponding analog signal back to the PCM. This type of sensor will have three wires: reference, signal and ground.
The other type of MAP sensor is the digital, or frequency type, which produces a digital voltage signal instead of an analog signal. A cut away of an analog MAP sensor As was mentioned, some vehicles use an independent BARO sensor. These devices are designed to measure only barometric pressure. The BARO sensor is similar to the MAP sensor in design but is more sensitive to changes in atmospheric pressure. The code P0069 stands for manifold pressure/barometric pressure correlation.
The PCM compares BARO sensor and MAP readings at idle. If the difference between the two is greater than specified, the PCM will set a diagnostic trouble code (DTC).