Technical Description of OBD-II Trouble Code
Low Barometric Pressure
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to all vehicles manufactured from 1996 onwards (Dodge, Chrysler, Hyundai, Jeep, Mazda, etc.). Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
When you encounter a stored P0129 code in your OBD-II equipped vehicle, it indicates that the powertrain control module (PCM) has detected a low reading of barometric pressure from either the barometric pressure sensor (BPS) or the manifold absolute pressure sensor (MAP).
The barometric pressure sensor allows the PCM to monitor changes in atmospheric pressure, which is directly related to the altitude at which your vehicle is operating. This information is used by the PCM to calculate fuel delivery and ignition timing strategy.
The barometric pressure sensor is typically supplied with a reference voltage signal, a battery ground, and one or more output signal circuits. The BPS completes the reference voltage circuit and provides the appropriate resistance based on the actual barometric pressure.
As atmospheric pressure changes, the resistance levels of the BPS react accordingly. Fluctuating resistance in the BPS leads to changes in the sensor’s output voltage to the PCM. These voltage signals, which change over time, are not only used by the PCM to map fuel delivery and ignition timing strategy, but also compared to programmed parameters for code storage.
Since automakers use different designs for monitoring barometric pressure, you will need to consult your vehicle information source (such as All Data DIY) to determine the exact BPS design of your vehicle. Most manufacturers use a separate BPS that is mounted away from the engine and typically has a vacuum hose connected to it.
Some models utilize a system in which the BPS is integrated with the MAP sensor. In this type of system, the term “manifold absolute pressure” is usually substituted for “barometric pressure” in the scanner data stream.
If the PCM detects a BPS input signal that deviates from the comparison data by more than the programmed limit, a P0129 code will be stored and the malfunction indicator lamp (MIL) may illuminate.
Severity & Symptoms
Symptoms of a P0129 code may include:
Causes
Possible causes for this code include:
Diagnostic and Repair Procedures
A good starting point is always to check for technical service bulletins (TSB) for your particular vehicle. Your issue may be a known problem with a known solution provided by the manufacturer, which can save you time and money during diagnosis.
When diagnosing a P0129, it is recommended to have a diagnostic scanner, a digital volt/ohmmeter (DVOM), a vacuum gauge, and a reliable vehicle information source (such as All Data DIY).
It is advisable to begin the diagnosis with a visual inspection of all system-related wiring harnesses and connectors. Any damaged wiring or corroded connector faces should be repaired before proceeding with the diagnosis.
Next, connect the scanner to the vehicle diagnostic connector and retrieve all stored codes. Make a note of all codes and freeze frame data, as this information may be useful if dealing with an intermittent code. Clear the codes and test-drive the vehicle to see if the code is reset.
If the code is reset, perform a manual engine vacuum test to ensure that the engine is generating sufficient vacuum. Consult the vehicle information source for specifications that apply to your vehicle. If the engine is not performing efficiently, it may not produce enough vacuum for the BPS or MAP sensor. Engine misfire codes, fuel system codes, and catalytic converter codes should be diagnosed and repaired before attempting to diagnose this code.
Keep in mind that barometric pressure sensors integrated with the MAF sensor are prone to corrosion at the connector face. These systems are also susceptible to faults caused by an extremely dirty air filter element. Additionally, this type of BPS configuration often requires the installation of an auxiliary ground (depending on the manufacturer).
If all circuits and connectors appear to be operational, continue testing at the BPS. Unplug the electrical connector and check the resistance across all circuits of the BPS using the DVOM. If the sensor does not comply with specifications, it should be replaced.
If the BPS meets the manufacturer’s specifications, proceed to test the system circuits. To prevent damage, disconnect all related controllers before testing the system circuits with the DVOM. Repair or replace any open or shorted circuits as needed.
If the sensor and all circuits are compliant with the manufacturer’s specifications, suspect a defective PCM or a PCM programming error. Consider all other possibilities before opting for replacement, and remember that replacing system controllers will require reprogramming.
Additional diagnostic notes:
