The P0108 diagnostic trouble code indicates that the powertrain control module has detected a high voltage condition in the Manifold Absolute Pressure (MAP) or Barometric Pressure (BARO) sensor circuit. This generic code applies to most OBD-II vehicles, though exact sensor configurations vary.
A high circuit reading does not automatically prove the sensor has failed. The issue could stem from a short to voltage, an open ground, or wiring damage. Proper electrical testing is required to identify the root cause before replacing any components.
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Key Takeaways
- P0108 is a generic code indicating the control module detected an unexpectedly high voltage in the MAP or BARO sensor circuit.
- Possible causes include a short to power, an open sensor ground, damaged wiring, or a failed sensor element.
- Symptoms often include an illuminated Check Engine Light, poor engine performance, rich fuel mixtures, and hesitation during acceleration.
- Diagnosis requires verifying the exact sensor type and measuring circuit voltages against manufacturer specifications.
- Repairs depend on the confirmed fault and may involve wiring repair, connector service, or sensor replacement.
P0108 Code Description and Implications
The P0108 code indicates that the powertrain control module (PCM) has detected a voltage signal on the Manifold Absolute Pressure (MAP) or Barometric Pressure (BARO) sensor circuit that exceeds the normal maximum operating range.
Most MAP and BARO sensors operate on a 5-volt reference system. As manifold vacuum increases, the sensor’s signal voltage typically decreases. Conversely, when manifold pressure rises, the signal voltage increases.
A circuit high condition generally means the PCM is seeing a voltage near or at the 5-volt reference. The control module interprets this extremely high voltage as an impossibly low vacuum or high atmospheric pressure condition.
This false pressure reading can cause the PCM to miscalculate engine load, leading to incorrect fuel delivery, erratic ignition timing, and disrupted transmission shift scheduling.
Common Causes of the P0108 Code
Because P0108 is an electrical circuit code, the root cause usually involves the wiring, connectors, or the sensor itself.
- Short to voltage: The sensor signal wire may be rubbing against another circuit or a power source, feeding excessive voltage back to the PCM.
- Open ground circuit: If the sensor loses its dedicated ground path, the signal circuit can float high, triggering the code.
- Corroded or damaged connectors: Moisture intrusion or pushed-out pins at the sensor or PCM connectors can interrupt the circuit.
- Failed sensor element: An internal failure within the MAP or BARO sensor can cause it to output an incorrect high voltage.
- PCM issues: Rarely, an internal fault in the control module’s analog-to-digital converter may misread a normal signal as high.
Symptoms Associated With P0108
When the PCM detects a critical sensor fault, it may alter engine operation to protect the drivetrain and emissions system.
- Illuminated Check Engine Light: The malfunction indicator lamp will typically turn on immediately upon detecting the circuit fault.
- Poor engine performance: The engine may hesitate, stumble, or lack power during acceleration due to incorrect load calculations.
- Rich fuel mixture: The PCM may inject excess fuel, causing black exhaust smoke, fouled spark plugs, and decreased fuel economy.
- Rough idling or stalling: Incorrect air-fuel ratios at idle can cause the engine to run roughly or stall when coming to a stop.
- Transmission shifting issues: The PCM uses engine load data to schedule shifts; a false high-pressure reading can cause harsh or delayed shifts.
Diagnosing the P0108 Trouble Code
Accurate diagnosis begins by verifying the exact vehicle application and reviewing manufacturer-specific service information.
- Check live data: Use a scan tool to observe the MAP or BARO sensor reading. Compare the pressure or voltage reading to known atmospheric pressure with the key on, engine off.
- Visual inspection: Examine the wiring harness from the sensor to the PCM. Look for chafing, melted insulation near exhaust components, or corrosion at the connectors.
- Verify reference and ground: Using a digital multimeter, confirm that the sensor is receiving the proper 5-volt reference and has a clean, dedicated ground.
- Test the signal circuit: If the reference and ground are correct, test the signal wire for a short to voltage or an open circuit between the sensor and the PCM.
- Check for vacuum issues: On some applications, a completely blocked vacuum hose to the MAP sensor can cause it to read atmospheric pressure, though this usually triggers a rationality code.
Repairing the P0108 Issue
Repairs must follow the confirmed root cause identified during diagnosis.
- Wiring and connector repair: If a short to voltage or an open ground is found, repair or replace the affected harness section and clean any corroded terminals.
- Vacuum hose repair: If the MAP sensor uses a vacuum hose and it is disconnected, cracked, or blocked, replace the hose and ensure a tight seal.
- Sensor replacement: If the wiring, vacuum supply, and PCM circuits test correctly but the sensor output remains out of specification, replace the MAP or BARO sensor.
Cost Considerations for P0108 Repairs
The cost to resolve a P0108 code depends heavily on the underlying fault and the vehicle’s design.
If the issue is limited to a damaged wire, a disconnected vacuum hose, or a corroded connector, the repair may primarily involve diagnostic time and minor materials.
If the MAP or BARO sensor requires replacement, parts costs are generally moderate. Labor time varies based on sensor accessibility and whether the sensor is integrated into the throttle body.
Prevention Strategies for Future Occurrences
While electronic sensor failures cannot always be predicted, you can reduce the risk of circuit-related faults.
- Inspect vacuum hoses connected to the MAP sensor for signs of cracking, swelling, or looseness during routine engine bay inspections.
- Keep the engine bay reasonably clean to prevent excessive moisture and corrosive debris from accumulating around electrical connectors.
- Ensure that any aftermarket accessories are wired correctly and do not interfere with the factory engine wiring harness.
Troubleshooting Tips and Best Practices
Follow these practices to avoid misdiagnosis and ensure accurate repairs.
- Do not ignore driveability issues: A false high-pressure reading can cause the engine to run excessively rich, which may eventually damage the catalytic converter.
- Avoid parts swapping: Replacing the sensor without verifying the reference, ground, and signal circuits can result in a misdiagnosis if the wiring is actually at fault.
- Check technical service bulletins: Some manufacturers have published TSBs regarding specific wiring harness routing issues or updated sensor part numbers that address false P0108 codes.
- Understand sensor integration: On many modern engines, the MAP sensor is integrated directly into the throttle body assembly, requiring replacement of the entire assembly rather than just the sensor.
More OBD-II Codes
| P0060 | P0068 | P0087 | P0100 |
| P0101 | P0107 | P0111 | P0112 |
| P0113 | P0117 | P0118 | P0121 |
| P0130 | P0131 | P0132 | P0133 |
| P0135 | P0171 | P0172 | P0174 |
| P0300 | P0420 | P0440 | P0455 |
Frequently Asked Questions
Is P0108 a generic or manufacturer-specific code?
P0108 is a generic SAE code. Its core definition applies across all OBD-II compliant vehicles, though the exact sensor location and diagnostic steps vary by manufacturer.
Can a vacuum leak cause a P0108 code?
A vacuum leak typically causes a low-pressure reading, which might trigger a different code. P0108 specifically indicates an electrical high voltage condition, usually pointing to a short circuit or failed sensor.
Will disconnecting the battery clear the P0108 code?
Disconnecting the battery might temporarily clear the code. However, it does not repair the underlying electrical fault, and the code will likely return once the PCM runs its self-tests again.
Does P0108 affect the transmission?
Yes. The PCM uses manifold pressure data to calculate engine load and schedule transmission shifts. A false high-pressure reading can cause harsh, delayed, or erratic shifting.
Can I drive with a P0108 code?
Driving is not recommended. The PCM may inject excess fuel to compensate for the false load reading, leading to poor drivability, fouled spark plugs, and potential catalytic converter damage.
What Should You Do About the P0108 Code?
When a P0108 code appears, confirm the exact factory definition and system layout for your specific vehicle. Because this code involves critical engine load monitoring, driving with an active fault may result in poor fuel economy or unpredictable transmission behavior.
Follow the applicable manufacturer diagnostic procedure to test the relevant system and components. Once the root cause is confirmed, complete the necessary electrical repairs or component replacements before returning the vehicle to normal service.
