The manifold air pressure (MAP) sensor and barometric pressure (BARO) sensor are two of the most crucial sensors in your engine (often, they’re integrated into a single sensor). Just like any other vehicle component, it isn’t uncommon for these sensors to malfunction.
P0129 is just one of several engine trouble codes related to an issue in these sensors. Read on to learn more about this trouble code.
What Does the P0129 Code Mean?
Diagnostic trouble code (DTC) P0129 stands for “Barometric Pressure Too Low.” It is logged once the powertrain control module (PCM) perceives a low pressure (below manufacturer-specified limit) reading from either the MAP or BARO sensor.
The main function of a MAP sensor is to monitor engine load. It’s designed to react to pressure changes in the engine manifold. The voltage signal from this sensor is expected to vary as the engine load changes. An engine that is under load is expected to have greater pressure than a coasting engine.
A BARO sensor works just like a MAP sensor. The only difference is that BARO sensors are designed to detect more subtle changes in atmospheric air pressure. In many cases, the two sensors are integrated together.
A MAP sensor is usually connected to the manifold, while a BARO sensor is vented directly to the atmosphere.
These sensors use voltage signals to communicate with the PCM. A BARO sensor is typically supplied with a reference voltage, a battery ground, and output circuit(s). Resistance in the BARO sensor is expected to vary depending on the changes in atmospheric pressure. This variation in resistance causes the sensor’s voltage output to change.
The ECM/PCM uses this data for correct fuel injection and ignition timing. Once the PCM perceives that the voltage signal from the BARO/MAP sensor is too low, it will trigger the P0129 code.
Make sure that you understand how MAP and BARO works before attempting any DIY troubleshooting of code P0129.
Note: The definition of code P0129 may be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.
What are the Possible Causes of the P0129 Code?
Here are some possible triggers of this engine code:
- Wiring or connector issues
- Lack of engine vacuum (due to engine wear, ignition misfire, or clogged cat-con)
- Faulty MAP or BARO sensor
- Malfunctioning PCM
What are the Common Symptoms of a P0129 Code?
Here are the possible symptoms of a P0129 code:
- Activated check engine light
- Poor engine performance
- Engine hesitation (upon acceleration)
- Increased fuel consumption
How to Diagnose the P0129 Code
Diagnosing a P0129 code can prove challenging. Given that there are many listed triggers of this error code, it may be difficult for you to pinpoint what is really causing the issue. If you don’t have the appropriate skills and tools for the job, it’s best to take your vehicle to an auto repair shop. A mechanic would know how to properly diagnose the issue.
If you’re an experienced DIYer and you want to try to diagnose this code yourself, we recommend consulting vehicle-specific repair manuals or online repair databases. Most of the time, these resources contain helpful information on how to diagnose vehicle issues.
How to Fix the P0129 Code
There is no one fix for a P0129 code. BARO and MAP sensor designs may vary significantly per vehicle manufacturer. For instance, repair steps for a P0129 code on a Jeep Grand Cherokee may not work for a code P0129 on a Chrysler Pacifica.
If you plan on fixing the underlying issue on your own, we recommend consulting a vehicle-specific repair manual. This is a reliable source of information, which can help you determine the appropriate repair procedure for sensor malfunctions. Alternatively, you may subscribe to an online repair database.
MAP, BARO, and OBD Codes
Air pressure at sea level is 14.7 psi, which is equivalent to 29.92 Hg (inches of mercury measured using a specially designed graduated scale on a tube filled with mercury).
Manifold absolute pressure is measured on the Hg scale so that MAP is the opposite of manifold vacuum. Thus, when you add manifold vacuum and manifold pressure together, you get barometric pressure. For example, if an engine has 18 inches Hg of vacuum and ambient barometric pressure is 30 Hg, then the manifold absolute pressure is 12 inches Hg.
Barometric pressure is a significant factor in air-fuel calculations as well as ignition timing. With lower barometric pressure (BARO), less fuel is required but more time is needed for combustion, so timing is advanced and fuel delivery is adjusted accordingly. Typically, since the pressure in the manifold is the same as barometric (atmospheric) pressure when the engine isn’t running, the BARO pressure is measured between key on and engine start. On vehicles with a stand-alone BARO sensor (such as on supercharged and turbocharged vehicles), the BARO pressure is measured any time the key is on whether the engine is running or not.
And even on systems with an integrated MAP/BARO sensor, the BARO reading is updated during high-load Wide Open Throttle (WOT) events.
Engineers who write algorithms know this, and so they include algorithms to check the integrity of the sensor and its circuits, and it typically checks the MAP sensor with the key on and the engine not running. If the MAP reading falls below the acceptable range, a code like P0129 is set.
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