P0108

What Does Code P0108 Mean?

DTC P0108 indicates a condition where the Engine Control Module (ECM), also known as the Powertrain Control Module (PCM), detects an abnormally high input voltage from the Manifold Absolute Pressure (MAP) sensor or the Barometric Pressure (BARO) sensor circuit. The MAP sensor is a critical component that measures the absolute pressure within the intake manifold, providing real-time data to the ECM regarding engine load and vacuum conditions. When the engine is off and the key is in the ‘on’ position (KOEO), the MAP sensor typically functions as a BARO sensor, measuring ambient atmospheric pressure. The ECM uses this pressure data, along with other sensor inputs, to precisely calculate fuel delivery, ignition timing, and sometimes even transmission shift points.

A “high input” signifies that the voltage signal from the sensor to the ECM is consistently above the sensor’s specified operating range (e.g., exceeding 4.9V-5V reference, depending on the specific vehicle’s calibration). This abnormal signal suggests to the ECM that the engine is under a much heavier load than it actually is, or that it is operating at a much lower altitude with higher atmospheric pressure. The ECM interprets this erroneous data, leading to improper adjustments in fuel trim and ignition timing, which directly impacts engine performance and efficiency. This code is typically set when the signal voltage is static at its maximum threshold, or continuously exceeds it for a predetermined period.

Common Symptoms

• Check Engine Light (CEL) Illumination: The primary and most common symptom.
• Rough Idling or Stalling: Incorrect fuel mixture can lead to an unstable idle or cause the engine to stall.
• Hesitation or Lack of Power: Inaccurate load calculation results in poor engine response during acceleration.
• Poor Fuel Economy: The ECM may command an overly rich fuel mixture based on the false high-load signal.
• Black Smoke from Exhaust: A sign of excessive fuel delivery (rich condition).
• Hard Starting: Especially noticeable in cold weather or after the engine has been sitting.
• Increased Emissions: Due to improper combustion, the vehicle may fail emissions tests.

What Causes the Code P0108?

• Faulty Manifold Absolute Pressure (MAP) Sensor: The sensor itself has failed internally, causing it to send an excessively high voltage signal to the ECM.
• Short Circuit to Voltage in the MAP/BARO Sensor Signal Wire: The signal wire from the MAP sensor may be chafed or damaged, causing it to short to a 5-volt reference or even a 12-volt power wire within the wiring harness.
• Open Circuit or High Resistance in the Sensor’s Ground Wire: If the ground circuit to the MAP sensor is compromised, the signal voltage can “float” high, mimicking a high input.
• Corrosion or Damage in the MAP Sensor Connector: Poor electrical contact at the sensor or ECM connector due to corrosion or physical damage can lead to intermittent or consistently high voltage readings.
• Faulty Engine Control Module (ECM/PCM): While rare, an internal fault within the ECM itself can misinterpret the sensor’s signal or fail to provide the correct reference voltage, leading to a P0108.

How to Diagnose and Troubleshoot

Accurate diagnosis of P0108 requires a systematic approach using specialized tools such as an OBD-II scanner with live data capabilities and a Digital Multimeter (DMM).

1. Visual Inspection:
   • Begin with a thorough visual inspection of the MAP sensor, its electrical connector, and the associated wiring harness. Look for obvious signs of damage, such as frayed wires, rodent damage, corrosion at the connector pins, or loose connections.
   • Inspect the vacuum hose connecting the MAP sensor to the intake manifold (if applicable for a manifold-mounted sensor). Ensure it’s not cracked, collapsed, or disconnected.
2. OBD-II Scanner Live Data Analysis:
   • Connect an OBD-II scanner and access live data stream. With the Key On, Engine Off (KOEO), observe the MAP sensor’s voltage output or pressure reading (kPa/psi). At sea level, the MAP reading should closely approximate atmospheric pressure (e.g., around 101 kPa or ~4.5V to 4.9V, depending on sensor type and altitude). If the scanner displays a reading that is consistently at the maximum voltage (e.g., 5V) or significantly higher than expected atmospheric pressure, it confirms an electrical issue or faulty sensor.
   • Check for other related DTCs (e.g., P0107 for low input, P0106 for range/performance issues) which might provide additional diagnostic context.
3. Digital Multimeter (DMM) Testing at the Sensor Connector:
   • Disconnect the MAP sensor electrical connector.
   • Reference Voltage Check: With KOEO, back-probe the 5-volt reference wire at the harness connector (ECM side). Confirm a steady 5.0V (±0.2V) output from the ECM. If the voltage is significantly higher or absent, the ECM or wiring to the ECM is suspect.
   • Ground Circuit Check: Test the ground wire at the harness connector for good continuity to chassis ground (should be less than 0.5 ohms). Also, test for voltage on the ground wire; there should be none.
   • Signal Wire Voltage Check (Disconnected): With the sensor disconnected and KOEO, back-probe the signal wire at the harness connector. There should be minimal or no voltage present (ideally close to 0V). If you read 5V or higher, there is a short to voltage in the wiring harness between the sensor connector and the ECM.
4. Digital Multimeter (DMM) Testing at the Sensor (Connected):
   • Reconnect the MAP sensor electrical connector.
   • Back-probe the signal wire at the sensor connector (or directly at the sensor pins if accessible). With KOEO, observe the voltage. If it reads constantly at 5V or higher (confirming the P0108 condition) and the previous tests ruled out a short in the harness, the MAP sensor itself is faulty.
   • Vacuum Test (if applicable): If you have a hand-held vacuum pump, apply varying amounts of vacuum to the MAP sensor while monitoring the signal voltage with the DMM. The voltage should decrease smoothly and proportionally as vacuum increases. If the voltage remains high despite applied vacuum, the sensor is defective.
5. Wiggle Test: With the engine idling or KOEO and monitoring live data or DMM readings, gently wiggle the wiring harness and connector leading to the MAP sensor. Look for any fluctuations in the sensor’s voltage reading that could indicate an intermittent connection or a loose wire.

Recommended Repairs and Solutions

Once the root cause of P0108 has been precisely identified through diagnostic procedures, the following repairs and solutions are typically performed:

• Replace the MAP/BARO Sensor: If diagnostic tests confirm an internal fault with the MAP sensor (e.g., incorrect voltage output, no response to vacuum, or consistently high output), replacement with an OEM-equivalent part is the most common solution. Ensure the replacement sensor is correctly calibrated for your specific vehicle.
• Repair or Replace Damaged Wiring Harness: If a short to voltage, open circuit, or excessive resistance is found in the wiring leading to the MAP sensor, the affected section of the harness must be repaired. Use high-quality, heat-shrink butt connectors for splicing, or replace the entire harness section if damage is extensive. Proper insulation and strain relief are crucial to prevent future issues.
• Replace Corroded or Damaged Connector: If the electrical connector at the MAP sensor or ECM is corroded or physically damaged, it should be replaced. Ensure a tight, clean connection is established.
• ECM/PCM Replacement or Reprogramming: In rare cases where all other components and wiring have been thoroughly tested and confirmed to be functional, a faulty ECM may be the culprit. This is a complex and often expensive repair that usually requires reprogramming the new module to the vehicle. This should only be considered as a last resort after all other possibilities have been exhaustively ruled out.
• Clear DTCs and Test Drive: After performing any repairs, clear the P0108 DTC and any associated codes from the ECM using the OBD-II scanner. Then, perform a comprehensive test drive under various engine loads and speeds to ensure the fault does not return and that all readiness monitors complete successfully.