Comprehensive diagnostic guide for P0108

MAP/BARO Sensor Circuit High Input

Code and scope (for quick reference)

• Code: P0108
• Definition: MAP (Manifold Absolute Pressure) / Barometric Pressure sensor circuit high input. The sensor signal line is reading higher than the PCM expects for the given operating conditions.
• Code family context: P0108 is a powertrain diagnostic trouble code (P0xxx family) used within OBD-II systems. This grouping and code purpose are described in the OBD-II diagnostic trouble code references (Powertrain Codes section).
• Source note: The Open Source code definition explicitly identifies P0108 as the MAP/BARO sensor circuit high signal.

What This Code Means

• The PCM has detected the MAP/barometric sensor signal on the MAP circuit is higher than what the engine/load conditions would produce. This can be a true physical condition (actual high pressure/boost) or a fault in the sensor, wiring, or PCM interpretation.
• In practice, this may be accompanied by a Malfunction Indicator Lamp (MIL) and a stored DTC. Symptoms and causes can vary by engine design (NA vs. boosted/turbocharged) and by vehicle manufacturer's sensor characteristics.

Symptoms

• MIL illuminated with P0108 in the scan data
• Idle irregularities or rough idle
• Poor throttle response or reduced engine power
• Unexpected or high idle, or unstable idle
• Military-grade "feels like it's hitting a ceiling" acceleration or surging in some cases
• Possible fuel trim anomalies when diagnosed with a scanner (MAP reading not matching engine load)
• In boosted/turbo engines, you may observe higher live MAP readings than expected for any given load

Diagnostic Approach

1) Confirm the fault and gather data

• Use a scan tool to confirm P0108 is present and to capture freeze-frame data. Note engine rpm, load, temperature, barometric pressure reading (if available), MAP sensor scaling, and any other related sensor values at the time the code set.
• Review live data for MAP sensor voltage/pressure reading, MAP sensor ground status, and 5V reference line. If the vehicle provides kPa or inHg, compare with engine load.

2) Visual inspection and basic electrical checks

• Inspect MAP sensor and wiring harness for obvious issues: damaged insulation, cracked sensor, corroded or bent pins, loose connections, oil/chemical contamination, or presence of aftermarket wiring.
• Check the MAP sensor ground and the 5V reference line for continuity and proper voltage supply with the key on and engine off, and with the engine running (as applicable). Look for shorts to 5V or to ground.
• Inspect related connections for the barometric pressure sensor if it is separate from the MAP sensor.

3) Basic sensor and wiring testing

• Verify 5V reference at the MAP sensor connector with a known-good reference on the signal and ground lines. Readings should be stable and within spec for the vehicle (per service manual).
• With engine off, unplug the MAP sensor and observe the effect on the map-reading input (some vehicles show expected change when unplugged; others may show a fixed value; follow the service manual for the specific make/model).
• With ignition ON but engine OFF, observe MAP signal voltage. It should be at a defined baseline (not near the high end) per the vehicle's reference.

4) Compare MAP readings to actual manifold/barometric conditions

• For NA engines: compare MAP readings with a handheld vacuum/pressure source or with a known-good MAP sensor on the same engine to determine if the MAP value is abnormally high for the given load and RPM.
• For boosted engines: determine whether the MAP reading corresponds to the actual boost condition. If MAP shows abnormally high but the actual boost is normal, suspect sensor/wiring/PCM interpretation issues.

5) Vacuum and intake system check

• Inspect for unmetered air leaks or intake barrel leaks that could influence the MAP signal. While a vacuum leak generally reduces MAP (lower absolute pressure), some misinterpretations or sensor behavior can produce misleading readings depending on engine state and boost modulation.
• In turbo/SC applications, check for boost control leaks or venting that could cause unexpected MAP signal behavior.

6) Component-specific checks and tests

• MAP sensor: swap in a known-good MAP sensor if available to see if the fault follows the sensor. If the high signal persists with a known-good sensor, the problem is likely wiring or PCM-related.
• Wiring harness: check for damaged wires, pin corrosion, moisture intrusion, or any aftermarket devices that could inject voltage into the MAP circuit.
• Ground integrity: ensure the PCM ground and MAP sensor ground are solid; a marginal ground can cause anomalous sensor readings.
• 5V reference integrity: verify that the reference line is not being loaded by another device or shorted to supply.

7) PCM/ECU considerations

• If all sensor, wiring, and vacuum sources test OK and a replacement MAP sensor does not resolve the issue, consider PCM fault or a software/firmware anomaly. This is typically a last resort after all wiring and sensor checks.

8) Repair steps (typical sequencing)

• If MAP sensor is faulty or signal short to 5V/near-5V:
  • Replace MAP sensor with a new or known-good unit.
  • Re-test MAP signal voltage and system behavior after replacement.
• If wiring/harness issues are found:
  • Repair or replace damaged wiring, repair harness connectors, and ensure a solid ground and reference connection.
  • Re-test after repairs.
• If vacuum/boost leaks or unmetered air issues are found:
  • Repair leaks, replace damaged hoses or gaskets, and ensure proper intake sealing.
• If PCM fault is suspected:
  • Verify with service bulletin and diagnostic procedures for the specific vehicle; reflash or replace PCM as recommended by the manufacturer if indicated after sensor/wiring tests.

9) Verification and validation

• After repairs, clear the DTCs and perform a road test under typical operating conditions to ensure P0108 does not reoccur.
• Monitor live data to confirm MAP signal remains within expected range for engine load, and that 5V reference and ground are stable.
• Confirm that other related codes (MAP-related or pressure-related codes) are not present.

Safety Considerations

• Follow standard workshop safety: disconnect the battery when performing certain electrical tests, and avoid shorting power/ground pins.

• Use proper PPE when working around air intake systems and when performing smoke/pressure tests (where applicable).

• Be mindful of engine bay hot surfaces and moving parts when performing live data checks and sensor replacements.

• OBD-II diagnostic trouble codes and the concept of Diagnostic Trouble Codes are described in the Wikipedia OBD-II sections (Diagnostic Trouble Codes; Powertrain Codes). This supports the understanding that P0108 is a powertrain-level code used by the system to indicate a MAP/barometric sensor circuit anomaly.

• Practical interpretation and diagnostic steps described here align with general MAP sensor operation and OBD-II diagnostic logic described in the above sources. Real-world symptom descriptions reflect typical customer reports and standard engine-management behavior when a MAP signal is compromised.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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