DTC code P0129 OBD2 is stored because the Powertrain Control Module (PCM) has detected a barometric pressure reading below the set limit. This reading comes from the BPS (Barometric Pressure Sensor) or MAP (Manifold Absolute Pressure Sensor).
The BPS Sensor helps the PCM change to atmospheric pressure. This pressure is exactly related to the degree of altitude applied to the vehicle. The information obtained is used to calculate fuel delivery as well as ignition timing. Usually this sensor has a reference signal, a battery ground and an output signal circuit. In addition, this sensor completes the reference voltage circuit and delivers the necessary resistance as determined by the actual barometric pressure.
When there are changes in atmospheric pressure, the resistance levels of the BPS Sensor also react accordingly. This varying resistance in the sensor causes changes in the output voltage from the sensor to the PCM. In case the PCM finds an input signal from the BPS Sensor which varies from the programmed limit, this code will be stored.