What Does P0109 Code Mean?
OBD-II Code P0109 is defined as a Manifold Absolute Pressure Sensor/Barometric Pressure Circuit Intermittent
The Manifold Absolute Pressure Sensor/Barometric Pressure Sensor measures the rise and fall of the air pressure inside the Intake Manifold. This provides critical data needed for the Powertrain Control Module(PCM) to control the Air Fuel Ratio, the Ignition Spark Timing and many components of the Emission Control Systems.
Code P0109 sets when there is inconsistent or erratic performance in the voltage signal to the PCM from the MAP/Baro Sensor Circuit
P0109 Symptoms
- Check Engine Light will illuminate
- In some cases, the engine may be hard starting and/or get poor fuel economy
- In some cases, noticeably poor engine performance may occur
Common Problems That Trigger the P0109 Code
- Manifold Absolute Pressure Sensor/Barometric Pressure Sensor is defective
- Intake Manifold vacuum leaks
- Faulty or corroded Manifold Absolute Pressure Sensor/Barometric Pressure wiring or connections
Symptoms
- Check Engine Light will illuminate
- In many cases, no abnormal symptoms may be noticed
- In some cases, the engine may be hard starting and/or get poor fuel economy
- In some cases, noticeably poor engine performance may occur
Common Problems That Trigger the P0109 Code
- Manifold Absolute Pressure Sensor/Barometric Pressure Sensor is defective
- Intake Manifold vacuum leaks
- Faulty or corroded Manifold Absolute Pressure Sensor/Barometric Pressure wiring or connections
Common Misdiagnoses
- Manifold Absolute Pressure Sensor/Barometric Pressure Sensor is replaced when the real cause is a vacuum leak
- MAP Sensor is replaced when the real issue is a problematic EGR Valve
- Manifold Absolute Pressure Sensor/Barometric Pressure Sensor is replaced when the real problem is a defective Idle Air Control Motor, which causes a very low engine idle
Polluting Gases Expelled
- HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
- CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
- NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog
P0109 Diagnostic Theory for Shops and Technicians
When diagnosing a P0109 code, it is important to record the freeze frame information and then to duplicate the code setting conditions with a test drive while paying close attention to engine load, throttle position, RPM, and road speed on a data streaming scan tool. As you drive the vehicle, compare these values to the MAP Sensor PID or parameter ID. The MAP Sensor voltage values should rise and fall with changes in engine speed and engine load. Typically the values will vary from 4 volts or more when accelerating to 1 volt or less when decelerating.
Check the MAP sensor connector with the key on and the engine off. There needs to be a steady 5 volt reference voltage and a very good ground. The third wire will be the MAP Sensor signal wire which sends data to the Power Train Control Module. Find and use the proper engine performance wiring diagram to discern the proper color and position of these wires in the connector.
It never hurts to do a key on-engine off vacuum pump voltage output test of the MAP sensor. You should watch the output on the scan tool as this will verify the harness and connections as well. The voltage should steadily decrease from 5 volts to less than 1 volt as you apply 18-20 inches of vacuum. I also like to wiggle the MAP sensor wiring harness as I apply vacuum to ensure that the MAP sensor wiring and/or connector aren't contributing to the problem. Always inspect the MAP Sensor vacuum hose and if the MAP sensor plugs directly into the intake manifold, be sure to check its seal, as they can split and cause erratic readings.
