P0129 Barometric Pressure Too Low

P0129 indicates barometric pressure is too low. The controller sees a pressure value lower than expected for altitude and operating conditions.

Code details

Generic OBD-II · P0

System: Fuel / Air
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace the sensor before comparing the pressure value to actual local barometric pressure.

Symptoms of P0129

Check-engine light on, sometimes with rough or uneven running.
Hesitation, surging, or a noticeable change in fuel economy.
Occasionally hard starting or a smell of unburnt fuel.

Common causes of P0129

A failing or contaminated sensor (MAF, MAP, oxygen or air-fuel sensor).
Intake or vacuum leaks, or an exhaust leak near an upstream sensor.
Wiring, connector, or fuel-delivery problems that skew the reading.

Useful checks before replacing parts

Compare BARO reading with local atmospheric pressure, MAP key-on engine-off value, altitude, weather, and scan-tool units.
Inspect MAP or BARO sensor signal, reference, ground, connector condition, intake restrictions, clogged ports, and related boost codes.
Check whether the fault appears after altitude changes, sensor replacement, intake service, or low battery voltage events.

Related OBD-II codes

These nearby fuel / air entries can help compare wording and pin down the fault area.