P0108 - OBD-II Trouble Code

Auto Systems and Repair

Manifold Absolute Pressure/Barometric Pressure Circuit High Input

Our emissions expert has put together the following information about the P0108 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.

OBD II Fault Code

  • OBD II P0108

Fault Code Definition

  • Manifold Absolute Pressure Sensor/Barometric Pressure Circuit High Input

What does this mean?

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 P0108 sets when the voltage to the PCM from the MAP/Baro Sensor Circuit stays above 4.5 volts for 10 seconds or more when the values from the Throttle Position, RPM and Oxygen sensors are changing.

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 P0108 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 an Intake Manifold vacuum leak
  • Manifold Absolute Pressure Sensor/Barometric Pressure Sensor is replaced when the real cause is a stuck-open 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

P0108 Diagnostic Theory for Shops and Technicians
When diagnosing a P0108 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.  


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