Internal failure of the altitude sensor within the engine control module (ECM).; Corrosion or damage to the ECM connector pins or wiring harness affecting the internal sensor signal.; Water intrusion into the ECM housing, causing internal circuit damage.; Voltage supply issues to the ECM, such as a weak battery or faulty ground, leading to internal sensor misreads.; Software corruption or firmware glitch in the ECM that affects sensor processing.; Extreme environmental conditions (e.g., very high altitude) that push the sensor beyond its calibrated range, though this is less common.

DTC codes / Volkswagen codes / P1387

P1387 · VolkswagenControl Unit Internal Altitude Sensor

Q: What does code P1387 mean?

Control Unit Internal Altitude Sensor. P1387 on Volkswagen vehicles indicates that the engine control module (ECM) has detected an internal fault within its own altitude sensor circuit, which is integrated into the control unit. The ECM compares the altitude sensor signal with other pressure and temperature inputs and sets this code when the internal reading is implausible or out of range, often due to a failure inside the module itself.

P1387 on Volkswagen vehicles indicates that the engine control module (ECM) has detected an internal fault within its own altitude sensor circuit, which is integrated into the control unit. The ECM compares the altitude sensor signal with other pressure and temperature inputs and sets this code when the internal reading is implausible or out of range, often due to a failure inside the module itself.

SystemPowertrain

Code typeVolkswagen specific

SeverityMedium

Fix difficultyProfessional

AreaIgnition system and misfire

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms.
Rough idle or hesitation during acceleration, especially at high altitudes.
Reduced engine performance or power, as the ECM may default to a fixed altitude value.
Poor fuel economy or increased emissions due to incorrect air-fuel mixture calculations.
In rare cases, the engine may stall or fail to start if the ECM cannot determine barometric pressure.
Often no symptoms besides the warning light, as the ECM substitutes a default altitude value.

Common causes, most likely first

Internal failure of the altitude sensor within the engine control module (ECM).
Corrosion or damage to the ECM connector pins or wiring harness affecting the internal sensor signal.
Water intrusion into the ECM housing, causing internal circuit damage.
Voltage supply issues to the ECM, such as a weak battery or faulty ground, leading to internal sensor misreads.
Software corruption or firmware glitch in the ECM that affects sensor processing.
Extreme environmental conditions (e.g., very high altitude) that push the sensor beyond its calibrated range, though this is less common.

How to diagnose and fix it

Verify battery voltage and ensure the charging system is functioning correctly, as low voltage can cause false internal sensor codes.
Inspect the ECM connectors and wiring for corrosion, water damage, or loose pins, and clean or repair as needed.
Use a Volkswagen-specific scan tool to check for related fault codes in other modules and to perform a functional test of the altitude sensor if available.
Check for technical service bulletins (TSBs) or software updates for the ECM that may address known internal sensor issues.
If the fault persists after basic checks, replace the engine control module and perform coding and adaptation with a factory scan tool.
After replacement, clear all fault codes and perform a road test to confirm the code does not return.

Before you replace the ECM for P1387

A weak or failing battery is a common cause of phantom internal sensor codes on Volkswagen models, so always load-test the battery before condemning the ECM.
Water damage from a clogged plenum drain or a leaking windshield seal can corrode the ECM internals and trigger P1387, so inspect the ECM housing for moisture signs.
The altitude sensor is integrated into the ECM and cannot be serviced separately; replacing the ECM is the only fix if the sensor itself fails.
P1387 may be stored alongside codes for the manifold absolute pressure (MAP) sensor or mass air flow (MAF) sensor, so diagnose those first to avoid misdiagnosis.
In some cases, a software update from the dealer can resolve false altitude sensor readings without hardware replacement.

Can I keep driving with P1387?

You can generally continue driving with P1387, but avoid sustained high-altitude driving or heavy loads because the ECM may use a default altitude value that compromises performance and emissions. If the engine runs roughly or stalls, have the vehicle inspected promptly to prevent further issues.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Volkswagen vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.