P0108: Manifold Absolute Pressure/Barometric Pressure Sensor Circuit High

P0108 is a diagnostic trouble code indicating: Manifold Absolute Pressure/Barometric Pressure Sensor Circuit High. This code relates to the fuel and air metering system. It has been reported in NHTSA complaints across 3 different vehicle models.

The Manifold Absolute Pressure (MAP) sensor measures the vacuum/pressure inside the intake manifold. At idle, the manifold has high vacuum (low pressure); at wide-open throttle, pressure approaches atmospheric. The ECU uses this reading along with RPM to calculate engine load and determine fuel injection timing and amount. Some vehicles use MAP instead of MAF, while others use both for cross-referencing. The MAP sensor typically uses a piezoresistive element that changes resistance with pressure changes.

The MAP sensor uses a piezoresistive silicon element that changes resistance in response to pressure changes. At idle with high manifold vacuum, the MAP sensor reads 1-2 volts. At wide-open throttle (atmospheric pressure), it reads 4-4.5 volts. Key-on engine-off should read near atmospheric pressure (~4.5V) — this is a quick sanity check. Some vehicles use both MAP and MAF sensors for cross-referencing accuracy; others use one or the other. MAP-only vehicles (older Chrysler/Dodge products) are particularly sensitive to MAP sensor issues.

The MAP sensor uses a piezoresistive silicon element that changes resistance in response to pressure changes. At idle with high manifold vacuum, the MAP sensor reads 1-2 volts. At wide-open throttle (atmospheric pressure), it reads 4-4.5 volts. Key-on engine-off should read near atmospheric pressure (~4.5V) — this is a quick sanity check. Some vehicles use both MAP and MAF sensors for cross-referencing accuracy; others use one or the other. MAP-only vehicles (older Chrysler/Dodge products) are particularly sensitive to MAP sensor issues.

**Understanding P0108 in Depth:** The MAP (Manifold Absolute Pressure) sensor reads the pressure/vacuum inside the intake manifold using a piezoresistive silicon chip that flexes with pressure changes. At idle, the engine creates high vacuum in the manifold (low pressure, typically 15-22 inches of mercury), and the MAP reads low voltage (1-2V). At wide-open throttle, manifold pressure approaches atmospheric (0 inches of vacuum), and the MAP reads high voltage (4-4.5V).

The ECU uses MAP data alongside RPM to calculate engine load — this is the speed-density method of fuel calculation, used instead of or in addition to MAF data. Some engines use MAP only (many older Chrysler/Dodge vehicles), some use MAF only (most modern vehicles), and some use both for cross-referencing accuracy.

**Step-by-Step Diagnosis:** (1) With key on, engine off, the MAP sensor should read near atmospheric pressure (~4.0-4.5V or 29-30 inHg on a scan tool). (2) At idle, MAP should drop to ~1.0-2.0V (15-22 inHg). If it doesn't change between KOEO and idle, the sensor or its vacuum connection is faulty. (3) Check the vacuum hose between the MAP sensor and intake manifold — a cracked, kinked, or disconnected hose is the #1 cause of MAP codes. (4) Check for a stuck-open EGR valve — EGR flow raises manifold pressure and can push MAP readings outside normal range. (5) Test the sensor: apply vacuum with a hand pump while monitoring voltage on a multimeter — voltage should drop smoothly as vacuum increases.

**Vehicle-Specific Notes:** Chrysler/Dodge vehicles with speed-density fuel systems (no MAF) are extremely sensitive to MAP sensor issues — the sensor is the primary input for fuel calculation. Ford vehicles typically use both MAP and MAF, providing redundancy. GM vehicles use MAP primarily for barometric pressure correction and as a backup to the MAF sensor. Honda MAP sensors are integrated into the throttle body on some models and are not separately replaceable.