What Does P0129 Mean?
The ECM has detected that the barometric pressure (BARO) reading is below the minimum expected threshold. The BARO sensor may be integrated into the MAP sensor or be a standalone sensor, and it helps the ECM calculate air density for fuel and timing corrections. An abnormally low reading could indicate a sensor fault or that the sensor is reading manifold vacuum instead of true barometric pressure.
Common Causes
35%
Faulty MAP/BARO sensor reading abnormally low barometric pressure due to internal calibration drift or diaphragm leak
25%
Vacuum leak at the BARO sensor reference port or hose, causing the sensor to read manifold vacuum instead of atmospheric pressure
20%
Restricted or kinked vacuum hose to BARO sensor preventing it from sampling true atmospheric pressure during key-on
20%
Wiring issue on BARO sensor signal circuit causing attenuated voltage reading at the ECM
Diagnostic Steps
1
Compare the BARO reading on a scan tool with the actual local barometric pressure (available from weather services). At sea level, BARO should read approximately 29.92 inHg (101.3 kPa). If significantly lower, the sensor or circuit is faulty.
2
Inspect the vacuum hose routing to the MAP/BARO sensor. Ensure it is connected, not kinked, and not routed to a manifold vacuum source instead of an atmospheric reference port.
3
Disconnect the vacuum hose from the BARO sensor and verify the reading changes to near atmospheric pressure. If it reads correctly with the hose disconnected, the hose is connected to a vacuum source.
4
Measure the BARO sensor output voltage at the ECM connector. With the sensor exposed to atmosphere, voltage should be approximately 4.0-4.5V (depending on altitude). Significantly lower voltage indicates sensor failure or wiring issue.
5
Check for other DTCs related to MAP sensor performance (P0106-P0109), as many vehicles use a combined MAP/BARO sensor that reads barometric pressure at key-on before the engine starts.
Estimated Repair Cost
$75 - $350
Parts + labor, varies by vehicle and location
The ECM has detected that the barometric pressure (BARO) reading is below the minimum expected threshold. The BARO sensor may be integrated into the MAP sensor or be a standalone sensor, and it helps the ECM calculate air density for fuel and timing corrections. An abnormally low reading could indic...
The most common cause of P0129 (Barometric Pressure Too Low) is: Faulty MAP/BARO sensor reading abnormally low barometric pressure due to internal calibration drift or diaphragm leak
Typical repair costs for P0129 range from $75 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
An incorrect barometric pressure reading causes the ECM to miscalculate air density, resulting in improper fuel mixture and ignition timing. This can lead to reduced power, poor fuel economy, and elevated emissions but does not pose an immediate safety risk.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0129 to identify the root cause.
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Quick Info
Category
Powertrain
System
Engine Management & Air Metering System
Difficulty
Type
Generic (SAE)
Recommended Tools
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