|Code||Fault Location||Probable Cause|
|P0108|| Manifold absolute pressure (MAP) sensor/barometric pressure (BARO) sensor -high input |
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|Wiring short to positive, MAP sensor, BARO sensor, ECM|
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What Does Code P0108 Mean?
OBD II fault code P0108 is defined as “Manifold Air Pressure/Barometric Pressure Circuit High Input”, and is set when the PCM (Powertrain Control Module) detects a signal from the MAP (Manifold Air Pressure Sensor) that falls outside of the expected value for the reigning engine load or throttle position, or otherwise fails to correlate with the actual pressure/vacuum in the intake manifold.
All applications use data from the manifold pressure (or vacuum) to calculate an appropriate fuel delivery strategy to suit the current engine load, and to make suitable adjustments to ignition timing as well as valve/cam timing on applications where VVT or VCS systems are present. From this it should be obvious that incorrect or invalid data can cause serious disruptions in the smooth and efficient operation of an engine.
On most applications, a fault code is set when the PCM detects a mismatch between the throttle position and the signal voltage from the MAP sensor for more than four continuous seconds. The deviation from the correct sensor voltage is measured against a reference voltage for the particular application. However, the four-second code setting parameter may differ between applications, which makes it important to consult the manual for the vehicle being worked on for accurate information on code setting parameters.
Note: Circuit High Input codes are mostly caused by defective alternators that develop excessive voltages, although there may be other causes as well, such as short circuits that “leak” current into a particular system from other, unrelated systems that work on higher voltages. Note that in the case of short circuits that leak power into a control circuit, there is likely to be other, seemingly unrelated trouble codes present along with the code(s) being investigated. Diagnosing a “high input” code will always involve a thorough testing of the charging system as a first step, followed by measures to isolate the system from all other possible sources of power during resistance, continuity, and reference voltage tests.
What are the common causes of code P0108 ?
As with symptoms, the common causes of code P0107 and its closely related codes, P0108 and P0109 much the same on all applications. Note however, that one common cause, a poorly running engine, could set other codes along with P0107, P0108,and P0109, depending on the particular application. Below are some common causes this group of codes-
Lean running conditions caused by unmetered air entering the engine
Misfiring conditions caused by a poorly maintained engine
Clogged or damaged catalytic converter(s)
Defective MAP sensor
Defective MAF sensor(If fitted)
Defective throttle position sensor
Defective barometric sensor (If fitted)
Shorted, burnt, or damaged wiring and connectors
Defective PCM (Rarely)
What are the symptoms of code P0108 ?
While the symptoms of code P0107/P0108/P0109 are much the same on all applications, it should be noted that not all symptoms are always present on all applications. Moreover, the severity of some symptoms may vary between applications, but for the most part, the most common symptoms include the following-
Illuminated CHECK ENGINE light
Stored trouble code(s), which may on occasion include make and model specific codes
Increased fuel consumption
Decreased engine power and performance
Hesitation upon acceleration in the majority of cases
Unpredictable engine stalling
How do you troubleshoot code P0108 ?
NOTE #1: Some applications use MAF (Mass Airflow Sensors) instead of manifold pressure sensors. Moreover, some applications use a barometric sensor that is incorporated into the MAF sensor, with the barometric sensor serving as a back-up to the MAF sensor. In these cases, it is important to inspect the heated element of the MAF sensor for the presence of grime or residues that could impede the performance/accuracy of the sensor. In many cases, a simple cleaning of the element with an approved solvent will cure the problem.
NOTE #2: Before attempting to diagnose code P0107/P0108/P0109, it is important to ensure that the engine is in perfect running order, with no lean running or misfire conditions present. For instance, unmetered air entering the engine could interfere with the way in which the PCM interprets changes in the manifold pressure/vacuum. Inspect the vacuum system and repair all faults before attempting to diagnose this code. Also, be sure to scan for all trouble codes present, and to repair all other issues found before diagnosing this code.
NOTE: In order to make an accurate diagnosis, you will in addition to a suitable code reader and digital multimeter, need a repair manual for the application being worked on to determine the color-coding, location, purpose, and routing of all associated wiring.
Assuming that all other issues had been resolved, record all trouble codes present, and record all available freeze-frame data. This data will be of use if an intermittent fault is identified later on.
As a first step in the diagnostic procedure, check for a clogged or dirty air filter element, pinched, damaged, or dislodged vacuum hoses and inlet air ducting, as well as clogged or damaged catalytic converter(s). Repair/replace components and hoses as required. Operate the vehicle, and rescan the system to see if the code returns.
If however the above components are all in good order, perform a thorough inspection of all wiring and connectors. Repair/replace all burnt, corroded, damaged, or suspect wiring and connectors. Rescan the system to see if the code returns.
If the code returns, perform ground, continuity, and reference voltage checks on all wiring at the sensor connector. Be sure to disconnect all control modules before doing continuity checks to prevent damaging the modules, and consult the manual on the color-coding and function of each wire in the connector.
Compare all readings to values stated in the manual, and repair/replace wiring as required. Reconnect all wiring, and operate the vehicle. Rescan the system to see if the code returns.
If the code and symptoms persist, consult the manual for the correct test procedure(s) of the MAP/MAF sensor. Be sure to compare obtained readings to the manufacturer’s pressure to Hertz chart. Replace the sensor if any reading(s) fall outside of specified values. Operate the vehicle, and rescan the system to see if the code returns.
NOTE: If the vehicle is fitted with a MAF (Mass Airflow Sensor) instead of a MAP sensor and you have not already done so, now would be a good time to inspect the MAF sensor’s element for residues that could affect its performance. Only use approved solvents to (carefully) clean the element. Reassemble the inlet ducting, operate the vehicle, and rescan the system to see if the code returns.
If all obtained readings fall within the manufacturers specifications, it is likely that there is an intermittent fault present, which can sometimes be extremely challenging to locate and repair. If all other repair attempts fail to resolve the problem, you may have to allow the fault to worsen before an accurate and definitive diagnosis/repair can be made.
Although PCM failure is not altogether impossible, it is a rare event. Therefore, the fault must be sought elsewhere before any control module is replaced.