P0129: Barometric Pressure Too Low
The barometric pressure reading is lower than the engine control unit accepts.
Normal risk
Usually safe to drive. Have it checked at the next opportunity.
Short answer · Can I keep driving with P0129?
Usually yes. The engine may run slightly less smoothly or limit boost on turbo engines. Have it checked at the next opportunity.
What does P0129 mean?
P0129 means the engine control unit reads an outside air pressure lower than any real value at the altitudes it expects. Many engines take this reading from the manifold pressure sensor with the engine off, or from a sensor inside the control unit. A faulty sensor, a blocked sensor hose, a wiring fault or a control unit fault are typical causes.
What the car does when the code is set
The check engine light comes on. The engine uses a substitute value, so it may hesitate, idle unevenly, use more fuel or produce black smoke. On turbo engines, power may be limited.
How the system works
The manifold absolute pressure (MAP) sensor measures the pressure, or vacuum, in the intake manifold. Low pressure means low engine load; high pressure means high load or boost. The engine control unit uses it to calculate load and fuel quantity, and on turbo engines to control boost. The sensor sends a voltage that rises with pressure; with the engine off, the reading should match outside air pressure.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Split or disconnected vacuum hoseThe sensor reads manifold pressure through a small hose or port; a leak or loose hose gives a wrong reading.
- Intake leakAir leaking past a gasket or hose changes manifold pressure and the fuel calculation.
- Dirty or oily sensorOil vapour from the crankcase ventilation can coat the sensing element.
- Wiring or connector faultA broken wire, corrosion or a poor reference voltage changes the signal.
- Failed sensorThe sensor itself can drift or fail with age.
How it is diagnosed
- Check the hose and connectorMake sure the sensor is firmly connected and any vacuum hose to it is intact and not blocked.
- Compare the reading with the engine offWith the ignition on and the engine stopped, manifold pressure should be close to the barometric pressure for your altitude.
- Look for intake leaksA smoke test finds leaks in the intake manifold, hoses and gaskets.
- Measure supply, ground and signalA workshop checks the reference voltage and signal at the connector against the service data.Do not pierce wire insulation to take readings; it lets moisture in and causes new faults.
Questions about P0129
Can driving in the mountains set P0129?
Normal mountain roads should not set it, because the limits allow for high altitude. If it appears, check the sensor and its hose.
Is the MAP sensor the same as the MAF sensor?
No. The MAP sensor measures pressure in the intake manifold, the MAF sensor measures the air flow entering the engine. Some engines use both.