The P0108 trouble code is an OBD-II diagnostic code that indicates an issue with the Manifold Absolute Pressure (MAP) sensor or its circuit, often affecting engine performance and fuel efficiency. This article provides a detailed examination of P0108, covering its definition, causes, symptoms, diagnostic steps, and solutions. Supported by real-world case studies—such as issues in the Jeep Cherokee and other models—this guide offers practical insights for vehicle owners and technicians to effectively resolve this fault.

1. What is P0108?

P0108 is an OBD-II diagnostic trouble code (DTC) defined as "Manifold Absolute Pressure/Barometric Pressure Circuit High Input." It indicates that the Engine Control Module (ECM) has detected a voltage signal from the MAP sensor that is higher than the expected range (typically above 4.5 volts), suggesting a circuit issue or sensor malfunction. In some vehicles, the MAP sensor also functions as a barometric pressure sensor, measuring atmospheric pressure before the engine starts.

Role of the MAP Sensor

The MAP sensor measures the pressure inside the intake manifold, which varies with engine load and throttle position. The ECM uses this data to calculate the air density and adjust the air-fuel mixture and ignition timing for optimal combustion. A high input signal can lead the ECM to miscalculate these parameters, triggering P0108 and potentially causing performance issues.

Common Trigger Conditions

• MAP sensor voltage exceeds the normal operating threshold.
• Wiring or connector issues in the MAP sensor circuit.
• A malfunctioning MAP sensor or ECM.

2. Potential Causes and Specific Issues of P0108

P0108 can stem from various factors. Below is a detailed list of common causes, supported by real-world examples:

• (1) Faulty MAP Sensor
  Issue: The MAP sensor is defective, sending an abnormally high voltage signal.
  Example: In a 2017 Jeep Cherokee, the MAP sensor failed internally, outputting a constant 4.8V signal (normal: 0.5-4.5V), triggering P0108.
  Cause: Sensor wear or exposure to contaminants.
• (2) Wiring or Connector Issues
  Issue: A short to voltage or open circuit in the MAP sensor wiring causes an elevated signal.
  Example: A 2016 Ford Mustang had P0108 due to a shorted signal wire in the MAP sensor harness, pushing the voltage to 5V.
  Cause: Wiring insulation damage from heat or abrasion.
• (3) Vacuum Leaks
  Issue: A vacuum leak near the MAP sensor causes incorrect pressure readings, leading to a high voltage signal.
  Example: A 2018 Toyota Corolla exhibited P0108 due to a cracked vacuum hose near the intake manifold, affecting MAP sensor readings.
  Cause: Aging hoses or loose connections.
• (4) Clogged or Dirty MAP Sensor
  Issue: Debris or carbon buildup on the MAP sensor affects its ability to accurately measure pressure.
  Example: A 2015 Honda Civic triggered P0108 because of a dirty MAP sensor, clogged with oil residue from a leaking valve cover.
  Cause: Poor maintenance or oil contamination.
• (5) ECM Malfunction
  Issue: The ECM misinterprets the MAP sensor signal or has an internal fault.
  Example: A 2019 Nissan Sentra showed P0108 due to an ECM software glitch, later resolved with a firmware update.
  Cause: Outdated software or ECM hardware failure.

3. Symptoms of P0108

Symptoms of P0108 are often related to engine performance and fuel mixture issues. Here are common signs and their associated causes:

• Check Engine Light On
  Symptom: MIL illuminates on the dashboard.
  Cause: ECM detects a high input from the MAP sensor circuit.
• Poor Engine Performance
  Symptom: Hesitation, surging, or lack of power during acceleration.
  Cause: ECM miscalculates air-fuel mixture based on incorrect MAP data.
• Rough Idle
  Symptom: Engine idles unevenly or stalls.
  Cause: Improper air-fuel mixture affecting combustion stability.
• Increased Fuel Consumption
  Symptom: Noticeable drop in fuel efficiency.
  Cause: ECM overcompensates with fuel due to perceived high manifold pressure.
• Black Exhaust Smoke
  Symptom: Visible black smoke from the tailpipe.
  Cause: Rich air-fuel mixture from incorrect MAP readings.

4. Vehicles Prone to P0108 and Case Studies

P0108 is common in vehicles with MAP sensors, particularly those with high mileage or in harsh environments. Below are notable examples:

• Jeep Cherokee (2014-2019)
  Issue: Failed MAP sensor.
  Symptom: Rough idle and poor acceleration.
• Ford Mustang (2015-2020)
  Issue: Shorted signal wire in the MAP sensor circuit.
  Symptom: Black smoke and MIL on.
• Toyota Corolla (2014-2019)
  Issue: Vacuum leak near the intake manifold.
  Symptom: Increased fuel consumption and hesitation.
• Honda Civic (2012-2017)
  Issue: Dirty MAP sensor.
  Symptom: Stalling and rough idle.
• Nissan Sentra (2013-2019)
  Issue: ECM software glitch.
  Symptom: MIL on with no noticeable performance issues.

5. Steps to Diagnose P0108

Diagnosing P0108 requires a systematic approach. Follow these steps:

Step 1: Read Trouble Codes and Data Stream
Tool: OBD-II scanner (e.g., CR MAX).
Action: Connect to the OBD-II port, retrieve P0108 and related codes, and monitor live data.
Key Data:

1. 1. MAP sensor voltage: Should be 0.5-4.5V, varying with engine load.
   2. Manifold pressure: Should match expected values (e.g., 30 kPa at idle).

Example: The Cherokee showed a fixed 4.8V reading from the MAP sensor.

Step 2: Visual Inspection
Checkpoints:

1. 1. MAP sensor and connector: Look for damage or contamination.
   2. Wiring harness: Check for shorts, frays, or loose connections.
   3. Vacuum hoses: Inspect for cracks or leaks near the intake manifold.

Example: The Corolla had a cracked vacuum hose near the MAP sensor.

Step 3: Test the MAP Sensor
Tool: Multimeter and vacuum pump.
Action:

1. 1. Measure voltage: Should decrease with applied vacuum (e.g., 4.5V at 0 kPa to 1V at 80 kPa).
   2. Check reference voltage: Should be 5V from the ECM.

Example: The Cherokee’s MAP sensor showed a constant high voltage.

Step 4: Check for Vacuum Leaks
Action:

1. 1. Use a smoke machine to detect leaks in the intake system.
   2. Spray carb cleaner around hoses to listen for RPM changes.

Example: The Corolla’s RPM surged when carb cleaner hit the cracked hose.

Step 5: Inspect the MAP Sensor for Contamination
Action:

1. 1. Remove the MAP sensor and check for oil, carbon, or debris.
   2. Clean with MAF/MAP sensor cleaner if dirty.

Example: The Civic’s MAP sensor was clogged with oil residue.

Step 6: Verify the ECM
Action:

1. 1. Test with a known-good MAP sensor.
   2. Update ECM firmware if applicable.

Example: The Sentra’s issue resolved after a software update.

6. Solutions and Implementation

Apply these repairs based on diagnostic findings:

• Replace the MAP Sensor
  Part: Jeep OEM MAP sensor (~$50).
  Steps: Disconnect the connector, remove the sensor, install a new one, and clear codes.
  Example: The Cherokee’s performance normalized post-replacement.
• Repair Wiring or Connectors
  Method: Solder shorted wires or replace damaged connectors.
  Example: The Mustang’s wiring repair restored normal voltage.
• Repair Vacuum Leaks
  Part: Toyota OEM vacuum hose (~$20).
  Example: The Corolla’s leak was fixed with a new hose.
• Clean the MAP Sensor
  Part: MAP sensor cleaner (~$10).
  Example: The Civic’s sensor worked after cleaning.
• Update ECM Software
  Tool: Dealer equipment.
  Example: The Sentra’s P0108 cleared after the update.

7. Repair Costs and Precautions

• MAP Sensor: $50-$150.
• Wiring Repair: $50-$150.
• Vacuum Hose: $20-$50.
• MAP Sensor Cleaning: $10-$30.
• ECM Update: $0-$150.

Precautions

• Ensure proper sealing of vacuum hoses to prevent future leaks.
• Use dielectric grease on connectors to prevent corrosion.

8. Preventive Measures

• Inspect vacuum hoses during routine maintenance.
• Clean the MAP sensor every 30,000 miles if exposed to oil or debris.
• Use high-quality air filters to reduce contamination.
• Keep ECM software updated with manufacturer releases.

Summary

The P0108 trouble code indicates a high input issue in the MAP sensor circuit, often due to a faulty sensor, wiring issues, or vacuum leaks. Through detailed case studies—like the MAP sensor failure in the Jeep Cherokee or the vacuum leak in the Toyota Corolla—and systematic diagnostics, this guide equips readers to diagnose and fix the problem efficiently. Addressing P0108 promptly improves engine performance, fuel efficiency, and emissions. For thorough diagnostics, tools like the iCarsoft CR MAX from www.icarsoft-us.com provide real-time data and code-clearing capabilities to maintain optimal vehicle operation.