Barometric Pressure Too Low
What does P0129 mean?
The P0129 code means the vehicle's onboard diagnostic system has detected: Barometric Pressure Too Low. This is a low severity code.
Common Symptoms
- Check Engine Light on
- Slightly reduced engine power at high altitude
- Minor fuel economy decrease
- Engine may run slightly rich
- No major drivability concerns in most cases
Probable Causes (Ranked by Likelihood)
- Faulty barometric pressure sensor or MAP sensor Very High
- Driving at very high altitude (above 8,000 feet) High
- Wiring issue in the barometric pressure sensor circuit Moderate
- Restricted air intake affecting pressure readings Moderate
- PCM calibration issue Low
Step-by-Step Diagnostic Procedure
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Note your current altitude. At sea level, barometric pressure should read approximately 29.9 inHg (101 kPa). At 5,000 feet, approximately 24.9 inHg (84 kPa). If the reading does not match your altitude, the sensor may be faulty.
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With key on engine off, check the MAP/BARO sensor reading on a scan tool. It should approximate local atmospheric pressure.
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If the vehicle has a dedicated barometric pressure sensor, disconnect it and check the 5V reference and ground at the connector.
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Compare the sensor reading to a known good barometric pressure source (weather report for your area) to confirm accuracy.
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Inspect the sensor and wiring for damage or corrosion. On some vehicles, the BARO function is integrated into the MAP sensor.
Frequently Asked Questions
Can I drive with code P0129?
Yes, you can drive normally. The PCM uses barometric pressure to adjust fuel delivery for altitude. A slightly incorrect reading will cause minor fuel mixture errors but will not damage the engine.
Will this code go away if I drive to a lower altitude?
If the code was triggered by genuinely high altitude driving, it may clear itself at lower elevations. However, if the code persists at low altitude, the barometric pressure sensor is likely faulty.
Do all vehicles have a separate barometric pressure sensor?
No. Many modern vehicles use the MAP sensor to read barometric pressure with the key on before the engine starts. Some vehicles have a dedicated BARO sensor, typically located in the engine bay or integrated into the ECM.