P0108 OBD2 Code Guide
P0108 is a sensors diagnostic trouble code. Use this hub to understand the plain meaning, the first checks to run, and which vehicle-specific guides may be more useful before replacing parts.
MAP/Barometric Pressure Circuit High Input
Code P0108 signifies that the Powertrain Control Module (PCM) has detected an excessively high voltage signal from the Manifold Absolute Pressure (MAP) sensor. This high voltage typically corresponds to a low vacuum condition (or high pressure) within the intake manifold, which the PCM interprets as a heavy engine load or wide-open throttle. However, if this signal is present under conditions where a high vacuum (low pressure) is expected, such as at idle, it indicates a discrepancy. This can lead to incorrect fuel delivery, poor engine performance, reduced fuel economy, and potentially other drivability issues. The MAP sensor measures the absolute pressure inside the intake manifold and sends this data to the PCM, which uses it to calculate engine load and adjust fuel injection and ignition timing. A consistently high input suggests either a faulty sensor, an electrical issue in the circuit, or a genuine low vacuum condition.
Code verification standard
P0108 is published with SAE J2012 / ISO 15031-6 code structure plus OBD2 P0108 diagnostic context.
First checks
- - Read all stored and pending codes before clearing anything.
- - Save freeze-frame data so you know speed, load and temperature when the code set.
- - Inspect wiring, connectors, fuses, hoses and obvious leaks tied to the named system.
- - Compare live data against realistic values before buying sensors or modules.
- - Clear the code only after repair and complete a drive cycle to confirm it does not return.
P0108 diagnostic path
- 11. **Visual Inspection & Vacuum Check**: Begin by visually inspecting the MAP sensor's electrical connector and wiring harness for any signs of damage, corrosion, or loose connections. Crucially, check all vacuum lines connected to the MAP sensor (if applicable) and the intake manifold for cracks, kinks, disconnections, or leaks. A significant vacuum leak can directly cause a high MAP reading.
- 22. **Scan Tool Data Analysis (KOEO & Idle)**: Connect an OBD-II scan tool and monitor the MAP sensor's live data. With the key-on, engine-off (KOEO), the MAP reading should closely match the ambient barometric pressure (e.g., around 29-30 inHg or 100 kPa at sea level). Start the engine and observe the MAP reading at idle; it should drop significantly (typically to 17-21 inHg or 57-70 kPa, depending on the engine). If the reading remains consistently high (near barometric pressure) at idle, it confirms the high input issue.
- 33. **MAP Sensor Voltage Test**: Using a multimeter, back-probe the MAP sensor's signal wire while the engine is running at idle. Compare the measured voltage to the manufacturer's specifications (typically 0.5V-1.5V at idle, increasing with throttle application). A consistently high voltage (e.g., above 4.5V) at idle strongly indicates a problem. Also, verify the 5V reference voltage and the ground circuit at the sensor connector to rule out power supply issues.
- 44. **Comprehensive Vacuum Leak Detection**: If the MAP sensor appears to be functioning correctly electrically but the reading is still high, perform a thorough inspection for vacuum leaks. Use a smoke machine or spray carburetor cleaner around the intake manifold gaskets, PCV valve and hoses, brake booster, and any other vacuum-operated accessories. Listen for changes in engine RPM or observe smoke escaping to pinpoint leaks.
- 55. **PCM Input Verification (Advanced)**: If all previous checks confirm the MAP sensor, wiring, and vacuum system are sound, but the high input persists, it may be necessary to verify the high voltage signal directly at the PCM connector. If the high voltage is present at the PCM input pin, and the sensor and wiring are confirmed good, the issue could potentially be an internal PCM fault, though this is a rare occurrence.
Verify before repair
- - Replace the faulty MAP sensor if diagnostic steps confirm it is providing an incorrect high voltage signal.
- - Repair or replace any damaged, corroded, or open wiring and connectors in the MAP sensor circuit.
- - Locate and seal any vacuum leaks in the intake manifold, vacuum lines, PCV system, or brake booster that are causing a genuine low vacuum condition.
- - Address any severe carbon buildup in the throttle body or intake manifold that might be impeding proper airflow and vacuum readings (less common for P0108 but can contribute).
Vehicle-specific pages to check next
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