What does it mean?
When a P0129 code is stored in your OBD-II equipped vehicle, it means that the powertrain control module (PCM) has detected a low barometric pressure reading from the barometric pressure sensor (BPS) or the manifold air pressure sensor (MAP). The barometric pressure sensor is designed to enable the PCM to monitor variations in atmospheric pressure. Atmospheric pressure is directly related to the particular degree of altitude, as it applies to your vehicle.
This information is received and used by the PCM to help determine strategy for fuel delivery and ignition timing. The barometric pressure sensor is typically supplied with a reference voltage signal, a battery ground, and one (or more) output signal circuit/s. As changes in atmospheric pressure occur, BPS resistance levels react accordingly. Changing resistance in the BPS creates variations in sensor output voltage to the PCM.
These fluctuating voltage signals are not only used by the PCM to map fuel delivery and ignition timing strategy but also compared to programmed specifications. If voltage signals fail to comply with the programmed specifications, during a set period of time under certain circumstances, a P0129 will be stored and a service engine soon lamp may be illuminated. Because almost every automaker uses its own system of monitoring barometric pressure, this type of code can be confusing.
Here is a brief explanation of some of the most common designs. Keep in mind that this is far from being an exhaustive list. Consult your vehicle service manual or a professional technical data service to determine the exact BPS design of your vehicle. Some manufacturers use a BPS that is totally separate from the MAP sensor. It is usually mounted on the firewall or inner fender and has a vacuum hose connected to it.
The degree of engine intake vacuum is gathered by the sensor and input to the PCM, where it is compared with the input signal from the MAP sensor, intake air temperature, and other factors to determine actual altitude (elevation above sea level). If the BPS input signal varies by more than a set limit, a P0129 will be stored. Other automakers choose to integrate the BPS with the MAP sensor.
In this type of system the sensor is normally mounted directly to the engine intake manifold and sealed with some type of o-ring. Vehicles with this type of system usually substitute manifold air pressure for barometric pressure and the scanner data stream may reflect this terminology. In yet another BPS design the sensor is integrated into the mass air flow (MAF) sensor housing.
This type of system is relatively effective but makes BPS replacement more expensive since the entire MAF sensor assembly must be replaced if the BPS is defective. Since a single electrical connector is used in this system, the diagnostic process can be challenging. MAP and BPS are normally listed separately in this type of a system. The PCM typically takes ambient temperature variations into account when calculating altitude.
Automotive barometric pressure sensors are very accurate (with a tolerance of approximately one-meter). As a reference, altimeter calculations used in global positioning systems have a resolution of about 20-meters.