What the P0108 Code Means and How to Fix It

P0108 Code Explained: What It Means, Common Causes, and How to Fix It

Quick Summary

The P0108 code means the engine computer is seeing an abnormally high voltage signal from the pressure-sensing system—specifically, a Manifold Absolute Pressure high input and/or a Barometric Pressure high input condition. In plain terms, the PCM believes intake or atmospheric pressure is much higher than it should be. The most common causes are wiring faults, sensor reference/ground problems, vacuum issues, or a failed sensor. Proper diagnosis focuses on live-data comparison, electrical checks, and vacuum verification—not on parts swapping. Fixing the root cause restores accurate load calculations, fuel delivery, and drivability.

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What the P0108 Code Means

When I see a P0108 code, I know the powertrain control module (PCM) has detected a signal voltage that’s higher than expected. That’s why the code definition centers on Manifold Absolute Pressure high input and, on some vehicles, Barometric Pressure high input. The PCM uses these pressure signals to calculate engine load. If the signal stays too high, the PCM assumes the engine is under heavy load even when it isn’t.

On many vehicles, the barometric reading is derived from the MAP sensor at key-on, engine-off. On others, there’s a dedicated BARO sensor. Either way, a persistent high signal triggers the P0108 code because the PCM can no longer trust the pressure data it needs to control fueling and timing.

Why High MAP or BARO Input Causes Problems

The Manifold Absolute Pressure high input condition signals to the PCM that intake pressure is near atmospheric or boosted—conditions that should only occur at wide-open throttle or on turbocharged engines under boost. A high Barometric Pressure input suggests altitude data that doesn’t match reality. When either input is incorrect, the PCM enriches the mixture and adjusts timing, often resulting in poor fuel economy, hesitation, or rough running.

Accurate pressure data is foundational. When the PCM believes pressure is high at all times, it overfuels, mismanages spark advance, and can even damage the catalytic converter over time.

Common Symptoms of a P0108 Code

A P0108 code can produce a range of symptoms. I commonly see rough idle, black smoke on acceleration, fuel smell from the exhaust, and noticeably worse MPG. Some vehicles hesitate or bog at takeoff because the PCM is commanding too much fuel. Others may surge at cruise.

Because this is a “high input” fault, the check engine light usually stays on rather than flickering. On turbocharged applications, drivability issues may be more pronounced if the PCM limits boost as a protective strategy.

The Most Common Causes of the P0108 Code

Despite popular belief, the sensor itself isn’t always to blame. In real diagnostics, wiring and reference problems top the list for a P0108 code. A short to voltage on the signal wire can force a Manifold Absolute Pressure high input reading even with the engine idling. Likewise, a poor ground can raise the signal voltage and cause a Barometric Pressure high input fault.

Vacuum problems matter too. A disconnected, cracked, or restricted vacuum hose to the MAP sensor can cause the sensor to read atmospheric pressure when it shouldn’t. That alone can trigger the P0108 code. Oil contamination inside the sensor or connector corrosion can also skew readings.

Less commonly, the MAP or BARO sensor fails internally and outputs a stuck-high voltage. PCM failures are rare and should be considered only after all circuit tests pass.

How the MAP and BARO Systems Work

The Manifold Absolute Pressure high input diagnosis makes sense once you understand the circuit. The PCM supplies a 5-volt reference and a ground to the sensor. The sensor outputs a variable voltage proportional to pressure. Low pressure (high vacuum) equals low voltage. High pressure equals higher voltage.

The Barometric Pressure high input reading works the same way, but it measures atmospheric pressure. The PCM compares these values continuously. If MAP voltage remains high when vacuum should be present—or if BARO is implausibly high—the P0108 code sets.

How I Diagnose a P0108 Code

I start with a scan tool. With the key on, engine off, MAP and BARO values should be nearly identical. If one is pegged high, that’s a clue. Then I start the engine. At idle, MAP should drop significantly due to the vacuum. If it doesn’t, I suspect a vacuum leak or a high Manifold Absolute Pressure input caused by wiring.

Next, I inspect the vacuum hose and fittings. After that, I move to electrical tests. I check the 5-volt reference, ground integrity, and signal voltage. If the signal wire shows near-reference voltage all the time, I look for a short to voltage. If unplugging the sensor drops the signal, the sensor is likely at fault. This methodical approach avoids misdiagnosing a high Barometric Pressure input as a sensor failure when it’s actually a wiring issue.

The sensor receives a reference voltage from the computer and the ground.

Depending on the type of MAP sensor, it can alter the returning voltage or generate a digital square wave back to the computer.

You can test a MAP sensor for correct barometric pressure readings with a digital multimeter. This is a typical voltage chart for a GM sensor, based on the altitude at your testing location.

You can also test the MAP while it is running. Connect a vacuum gauge to the intake manifold and compare the gauge reading to the voltage output from the MAP sensor.

How to Fix the P0108 Code

Fixing a P0108 code depends on what you find. Many repairs involve replacing cracked vacuum hoses, repairing chafed wiring, or cleaning corroded connectors. Restoring a solid ground or correcting a shorted signal wire often resolves both Manifold Absolute Pressure high input and Barometric Pressure high input complaints instantly.

If testing proves the sensor is faulty, replacement is straightforward. After repairs, I clear the code and verify that MAP and BARO readings respond correctly across idle, acceleration, and cruise.

Why You Shouldn’t Ignore a P0108 Code

Ignoring a P0108 code allows the engine to run with incorrect load calculations. Over time, that can foul spark plugs, dilute oil with fuel, and overheat the catalytic converter. Addressing Manifold Absolute Pressure high input and Barometric Pressure high input issues early prevents far more expensive damage later.

© 2012 Rick Muscoplat