Technical Description of OBD-II Trouble Code
Intermittent Circuit for Manifold Absolute Pressure/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 (Ford, Hyundai, Kia, Mazda, Mercedes Benz, etc.). Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
When I come across a stored P0109 code, I understand that it indicates that the powertrain control module (PCM) has detected an intermittent reading of manifold absolute pressure/barometric pressure from either the manifold absolute pressure sensor (MAP) or the barometric pressure sensor (BPS).
The purpose of the MAP sensor is to enable the PCM to monitor changes in atmospheric pressure by measuring absolute intake manifold pressure. Atmospheric pressure is directly related to the altitude at which the vehicle is operating. The MAP sensor typically receives a reference voltage (5-volt) signal, a battery ground, and one or more output signal circuits.
The resistance levels of the MAP sensor react to variations in absolute manifold pressure. These changing resistance levels in the MAP sensor result in variations in the sensor’s output voltage to the PCM. The PCM uses these voltage signals to calculate fuel delivery and ignition timing, and compares them with programmed specifications.
If the input voltage fails to meet the programmed specifications under certain conditions for a specific period of time, a P0109 code will be stored and the malfunction indicator lamp (MIL) may illuminate.
There are various systems for monitoring manifold absolute/barometric pressure. To determine the exact design of the MAP sensor in your vehicle, refer to your vehicle service manual or All Data (DIY).
An example of a MAP sensor
Some manufacturers use both a MAP sensor and a separate barometric pressure sensor. In such cases, the MAP sensor is mounted away from the engine and connected to a vacuum hose. The sensor measures the degree of engine intake vacuum and sends this information to the PCM, which compares it with the input signal from the barometric pressure sensor, intake air temperature, and other factors to determine the actual altitude (elevation above sea level). If the MAP sensor and BPS input signals differ by more than the specified degree, a P0109 code will be stored and the MIL will illuminate.
Most automakers integrate the MAP sensor with the BPS. In this type of system, the sensor is mounted on the engine intake manifold. The scanner data stream may only display manifold absolute pressure instead of both MAP and barometric pressure for this type of system.
Severity & Symptoms
Since the conditions that cause a P0109 code to be stored are likely to result in very poor fuel efficiency and engine drivability issues, it is important to address this code urgently.
Symptoms of a P0109 code may include:
Causes
Possible causes for this engine 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.
The tools that I would likely use to diagnose a P0109 are a diagnostic scanner, a digital volt/ohmmeter, a vacuum gauge, and a vehicle service manual (or an online service such as All Data DIY).
I prefer to begin my diagnosis by visually inspecting the wiring harnesses and connectors related to the system. Any corroded connector faces or damaged wiring must be repaired before proceeding with the diagnostic process.
Connect the scanner to the diagnostic connector and retrieve all stored codes and freeze frame data. Make a note of this information, as it may be useful if the P0109 code is intermittent. Clear the codes and test drive the vehicle.
An engine vacuum test can help ensure that the engine is generating sufficient vacuum. Refer to your vehicle information source for the minimum engine vacuum specifications. Poorly running engines will not produce enough vacuum for the MAP sensor. Ignition misfire codes should be diagnosed and repaired before attempting to diagnose a P0109. Clogged catalytic converters, restricted mufflers, and low fuel pressure may also contribute to insufficient engine vacuum.
If a P0109 code is immediately reset and all circuits and connectors appear to be in order, continue testing at the MAP sensor. Follow the manufacturer’s recommendations for testing the MAP sensor. Use the DVOM to test the resistance of the MAP sensor. If the MAP sensor does not comply with the resistance specifications, replace it. To prevent damage to related controllers, disconnect them before testing the resistance levels in the system circuits with the DVOM. Repair or replace any open or shorted circuits as necessary.
If the sensor and all circuits are compliant with the manufacturer’s specifications, suspect a defective PCM or a PCM programming error.
Other engine codes related to the MAP sensor include P0105, P0106, P0107, and P0108.
