Navigating car troubles can be daunting, especially when faced with cryptic error codes. If your OBD-II scanner is showing a P0108 code, you’re likely dealing with a “Manifold Absolute Pressure/Barometric Pressure Circuit High Input” issue. This article, crafted by the experts at autelfrance.com, will break down this error code, helping you understand the symptoms, potential causes, and effective solutions to get your vehicle running smoothly again.
Understanding the P0108 Code: MAP Sensor and High Voltage
The P0108 trouble code is a generic powertrain code, meaning it’s relevant for vehicles equipped with OBD-II systems. This code indicates a problem within the Manifold Absolute Pressure (MAP) sensor circuit. The MAP sensor plays a crucial role in your engine’s management system. It measures the pressure within the intake manifold – essentially, the engine vacuum. This information is relayed to the Powertrain Control Module (PCM), the brain of your car’s engine management system.
Most MAP sensors are three-wire sensors. These include a ground wire, a 5-volt reference wire from the PCM, and a signal wire. The signal wire communicates the MAP sensor’s voltage reading back to the PCM, and this voltage changes based on engine vacuum. Higher engine vacuum results in lower voltage, typically ranging from about 1 volt at idle to around 5 volts at Wide Open Throttle (WOT).
The P0108 code is triggered when the PCM detects an abnormally high voltage reading from the MAP sensor – specifically, a voltage exceeding 5 volts or a reading higher than expected under certain operating conditions. This “high input” signal alerts the system to a potential malfunction in the MAP sensor circuit.
Image alt text: Diagram of a typical Manifold Absolute Pressure (MAP) sensor, highlighting its electrical connector and vacuum port, crucial for diagnosing P0108 error.
Common Symptoms of a P0108 Error
When the P0108 code is active, you may observe several symptoms affecting your vehicle’s performance:
- Malfunction Indicator Lamp (MIL) Illumination: The check engine light will almost certainly turn on, indicating a detected issue.
- Poor Engine Performance: The engine might run rough, hesitate, or lack power due to incorrect air-fuel mixture calculations caused by faulty MAP sensor readings.
- Engine Stalling or Failure to Start: In severe cases, an incorrect MAP sensor signal can prevent the engine from starting or cause it to stall.
- Reduced Fuel Economy: Inaccurate pressure readings can lead to inefficient fuel combustion, resulting in decreased fuel mileage.
- Black Smoke from Exhaust: Rich fuel mixture, often a consequence of incorrect MAP sensor data, can cause incomplete combustion and black smoke emission.
Potential Causes Behind the P0108 Code
Several factors can lead to a P0108 error code. Here are some of the most common culprits:
- Faulty MAP Sensor: The MAP sensor itself could be malfunctioning, providing incorrect high voltage readings.
- Vacuum Line Leaks: A leak in the vacuum hose connected to the MAP sensor can disrupt pressure readings and cause the sensor to report incorrect data.
- Engine Vacuum Leaks: Leaks elsewhere in the engine vacuum system can also affect the MAP sensor’s readings, leading to a P0108 code.
- Short Circuit in Signal Wire: A short circuit in the MAP sensor’s signal wire to the PCM can cause an artificially high voltage reading.
- Short Circuit in Reference Voltage Wire: Similarly, a short in the 5-volt reference wire can also lead to an elevated voltage signal.
- Open Circuit in Ground Wire: An open or broken ground wire can disrupt the sensor’s operation and lead to incorrect readings.
- Worn Engine Components: In some instances, a worn-out engine with low vacuum can cause the MAP sensor to operate outside its normal parameters, triggering the P0108 code.
Diagnosing and Resolving the P0108 Error
Troubleshooting a P0108 code requires a systematic approach. Here’s a step-by-step diagnostic process:
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Initial Scan Tool Check: Use an OBD-II scan tool to confirm the P0108 code and check for any other related codes.
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MAP vs. Barometric Pressure Comparison (KOEO): With the key in the “Key On Engine Off” (KOEO) position, compare the MAP sensor reading on the scan tool with the Barometric pressure reading. These readings should be nearly identical, as both are measuring atmospheric pressure. If the MAP reading deviates by more than 0.5 volts from the BARO reading, a faulty MAP sensor is highly likely. Replacement of the MAP sensor might resolve the issue.
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Idle MAP Reading Check: If the KOEO test is inconclusive, start the engine and observe the MAP reading at idle. A normal idle reading is typically around 1.5 volts (this can vary based on altitude).
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a. Normal Idle Reading: If the idle reading is within the normal range, the problem might be intermittent. Carefully inspect all vacuum hoses connected to the MAP sensor for cracks, leaks, or damage. Replace any suspect hoses. Perform a wiggle test on the MAP sensor harness and connector to check for loose connections or wiring issues that could be causing intermittent faults.
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b. High MAP Reading (above 4.5 volts): If the scan tool shows a MAP reading exceeding 4.5 volts, use a vacuum gauge to measure the actual engine vacuum while the engine is running. If the vacuum is less than 15-16 inches of mercury (in. Hg), the PCM might be detecting insufficient vacuum for the engine’s operating condition. This could be due to engine wear or vacuum leaks. Address any engine vacuum issues and retest.
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c. High MAP Reading with Good Vacuum: If the engine vacuum is 16 in. Hg or higher, disconnect the MAP sensor. With the sensor unplugged and KOEO, the scan tool MAP reading should indicate no voltage. Check the ground circuit from the PCM to the MAP sensor for continuity and inspect the MAP sensor connector and terminals for tightness and corrosion. If the wiring and connections are good, replace the MAP sensor.
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d. Voltage Reading with MAP Unplugged (KOEO): If the scan tool still shows a voltage reading with the MAP sensor unplugged and KOEO, there might be a short circuit in the harness leading to the MAP sensor. Turn the ignition off. Disconnect the PCM connector and remove the MAP signal wire from the connector. Reconnect the PCM connector and check the MAP reading with KOEO again. If voltage is still present, the PCM itself might be faulty and require replacement. If the voltage reading disappears, check the signal wire you removed from the PCM for voltage. If voltage is present on this wire, there’s a short circuit in the harness. Locate and repair the short.
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Related OBD-II Trouble Codes:
- P0105: Manifold Absolute Pressure/Barometric Pressure Circuit Malfunction
- P0106: Manifold Absolute Pressure/Barometric Pressure Circuit Range/Performance Problem
- P0107: Manifold Absolute Pressure/Barometric Pressure Circuit Low Input
- P0109: Manifold Absolute Pressure/Barometric Pressure Circuit Intermittent
Need Further Assistance with P0108?
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Disclaimer: This information is intended for informational purposes only and should not be considered professional repair advice. We are not liable for any actions taken based on this information. Automotive repair should be performed by qualified technicians. All content is copyright protected.
