Fault code P0129 – barometric pressure too low

Fault code P0129 is called “Barometric Pressure (BARO) Too Low” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0129

Diagnostic Code P0129 indicates that the Powertrain Control Module (PCM) has detected a low barometric pressure reading. It is set when the barometric pressure sensor (BPS) or absolute manifold pressure sensor (MAP) reading is lower than the programmed reading.

Fault code P0129 – barometric pressure too low

Most Powertrain Control Modules (PCMs) rely on a varying number of measurements to accurately provide the engine with the optimum air/fuel ratio. Typically, Barometric Air Pressure (BARO) sensors are used when Absolute Manifold Pressure (MAP) sensors are also present.

BARO sensors are used to measure atmospheric pressure. This value is integral to determining fuel mixtures. Because the engine control unit needs to compare the atmospheric pressure with the pressure in the intake manifold. In order to fine-tune the fuel mixture to suit the needs of the driver during a particular period of travel.

In a fully functional system, the MAP and intake air temperature (IAT) sensors also provide input to the PCM.

The combined data (from all three sensors) ultimately determines the degree of throttle opening. As well as the volume and timing of the fuel supply to adapt to the ignition timing. This ensures that the engine runs at its optimum performance, at any given time.

The input value of the atmospheric pressure sensor (BARO) is compared with the programmed data. If the deviation from the expected value lasts for four consecutive seconds, the PCM will set code P0129 and the malfunction indicator light will illuminate.

Symptoms of vehicle malfunctions

The main signal that an error P0129 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. Check engine control lamp on the control panel will light up (the code will be recorded in the memory as a malfunction).
  2. The engine stops or starts badly.
  3. Floating revolutions, as well as attempts to stall at idle.
  4. Ignition skips under load or at idle.
  5. Reduced throttle responsiveness.
  6. Reduced engine power output.
  7. Fuel odor.
  8. Poor acceleration.
  9. Increased fuel consumption.
  10. In some cases, there may be no symptoms other than a stored fault code.

The error P0129 is serious, because when it appears, it can cause problems with the engine. But if there are no symptoms and only the Check Engine light is on, the OBD-II system can be restarted and the vehicle will continue to run normally.

Factors that can cause this error code

The error code P0129 can mean that one or more of the following problems have occurred:

  • Faulty or damaged barometric air pressure sensor (BARO).
  • Damaged or frayed connector of barometric air pressure sensor.
  • Damaged wiring.
  • Mechanical failure causing change in BARO readings.
  • Clogged catalytic converter.
  • Severe exhaust leaks in front of oxygen or NOx sensors.
  • Faulty throttle position sensor.
  • Faulty or contaminated mass air flow sensor (MAP).
  • Manifold pressure sensor has failed.
  • Faulty intake air temperature sensor (IAT).
  • Engine in poor mechanical condition or persistent ignition timing codes in one or more cylinders.
  • Sometimes the cause is a faulty PCM module.

How to fix or reset OBD-2 code P0129

Some suggested steps for troubleshooting and fix the error code P0129:

  1. Connect an OBD-II scanner to the vehicle’s diagnostic connector and read all stored data and error codes.
  2. Clear the error codes from the computer memory and test-drive the vehicle to see if code P0129 appears again.
  3. If the code appears again, check all cables and corresponding connectors leading to the BARO, MAP, MAF or TPS sensors. Make sure they are not broken or frayed. Repair or replace if necessary.
  4. Inspect and test these sensors.
  5. If necessary, replace the failed sensors.
  6. If faulty PCM is the cause, replace or reprogram it.

Diagnose and repair of problems

Inspect visible damage to sensors, wiring, and connectors. Then start the diagnostic procedure with the scan tool. To determine if error P0129 is the only error, if not, it may be worth investigating other fault codes first.

If no problems with damaged or corroded connectors are found, perform a thorough inspection of all associated wiring. Repair or replace damaged wiring as necessary.

Check for clogged air filter, damage to catalytic converter. Also look for damaged or disconnected air intake, dislodged or broken vacuum lines, and obstructions in the intake line. Replace hoses, air lines, filter element or vacuum lines if necessary.

Check BARO sensor

If the code remains, check the reference voltage at the BARO sensor connector. Also check the ground and continuity, especially in the signal wire between the PCM and the BARO sensor connector. If the resistance is infinite, repair the break in the atmospheric pressure sensor signal circuit.

If circuit continuity is normal, check the BARO sensor in operation by connecting the scan tool to the vehicle. The voltage from the BARO sensor should gradually decrease from 5 volts to 1 volt or slightly less, and increase back to about 5 volts. If the signal voltage does not change according to the manufacturer’s specifications, replace the sensor.

Often, BARO sensor readings can get stuck at 4.5 volts, regardless of engine operating conditions. This indicates a short between the signal wire and the 5-volt reference wire. If necessary, repair the wiring to clear the short.

If there is no voltage in the signal wire when the BARO sensor connector is disconnected, there is an internal short and the sensor must be replaced.

In some cases the PCM may be faulty or in the process of breaking down. But this is extremely rare. Therefore, before replacing it, it is better to check everything thoroughly again.

On which vehicles does this problem occur most frequently

Fault code P0129 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Chrysler (300, PT Cruiser, Pacifica, Town and Country)
  • Dodge (Caravan, RAM, Stratus)
  • Hyundai
  • Jeep (Grand Cherokee, Liberty, Wrangler)
  • Mazda
  • Mercedes-Benz
  • Subaru
  • Toyota
  • Volkswagen (Jetta)

Fault code P0129 can sometimes be found with other errors. The most common are the following: P0100, P0104, P0105, P0109, P0170, P0172, P0173, P0175, P0340.

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