OBD-II trouble code
P0129: Barometric Pressure Too Low
SAE J2012 names a barometric pressure that is too low. The 1999 Ford PCED, the 2004 L61 file, and the 2008 GRP09 sheet do not define P0129. Their low MAP and BARO codes are P0107, with different limits.
Quick facts
- System
- Powertrain
- Category
- Fuel & Air Metering
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $450
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0129 mean?
P0129 is Barometric Pressure Too Low in SAE J2012 APR2002. Too low is a plausibility judgment. The standard does not print the pressure that counts as too low, and it does not say the reading is impossible weather. Altitude and weather move barometric pressure. The calibration's minimum is what decides the code, and that minimum was not in the files searched for this page.
Those files do define a low pressure-sensor circuit, and they define it as P0107, not as P0129. On 2008 GM GRP09, P0107 is MAP voltage under 3 percent of the 5-volt reference, written as 0.15 volt, a short to low, an open, or the sensor. On the 2004 L61 file, P0107 is MAP under 0.05 volt, stated as 11.2 kPa. On the 1999 Ford PCED, P0107 is a BARO sensor whose PID is a frequency, failed below its calibrated minimum, with causes of an open or a short to ground and a reference fault, and with reference voltage that should be greater than 4.0 volts. None of those rows is titled barometric pressure too low. Using 0.15 volt, 11.2 kPa, or a frequency minimum as the P0129 limit would attach a circuit code's number to a different SAE title.
GRP09's evaporative monitor wants BARO at or above 74.0 kPa before it runs. That is an enable for a leak test, not a P0129 failure line. A vehicle can be below 74 kPa because it is high enough for that enable to suspend the leak test, and still be inside a plausibility window this page does not have a number for.
How the module learns barometric pressure is also calibration-specific. The 1999 Ford PCED treats BARO as its own sensor. Many MAP-only calibrations capture barometric pressure from the MAP sensor at key-on, before the engine pulls a vacuum. The diagram says which one this vehicle has. Comparing a MAP-derived BARO PID with the same MAP PID only proves the two labels share a transducer. It does not prove the reading is true. Compare a key-on reading with a local station pressure, in the units the scan tool uses, and then with the minimum printed in this vehicle's P0129 chart. This research did not find a single minimum that covers the code.
A restricted inlet or a MAP hose can move a pressure reading. Whether that movement is enough to be too low is the chart's question. The Ford P0107 causes are electrical. The L61 low-MAP wording is a short to ground or an out-of-range-low signal. Do not start with an air filter because a different code's name contains the word pressure.
Common causes
- The below-minimum barometric reading defined in this vehicle's own P0129 chart, which the searched GM and Ford files do not contain
- A MAP or BARO circuit that is actually failing P0107: under 0.15 volt on GRP09, under 0.05 volt on the L61 file, or the Ford frequency BARO low with reference not above 4.0 volts
- A VREF open or short to ground, which the 1999 Ford P0107 cause list names and which that chart checks as a reference fault rather than as P0129
- A key-on capture taken from a MAP sensor whose port is blocked or open to the wrong pressure, once the diagram shows that this calibration derives BARO from MAP
- A separate BARO sensor with a loose connection, which is the Ford P0109 cause and is not automatically this code
Symptoms
- Malfunction indicator lamp with P0129
- A barometric PID below the minimum in this vehicle's chart, compared with a local station pressure in the same units
- No P0129 row in the 1999 Ford PCED, the 2004 L61 file, or the 2008 GRP09 sheet
- P0107 stored instead, when the failure is the circuit low those three documents do define
- Fuel control using a default density, which these documents do not quantify as a mileage loss
Diagnostic steps
- 1.Find the P0129 minimum in the service information for this VIN. If the book has no such code and the stored code is P0107, use that procedure. GRP09 fails under 0.15 volt. The L61 file fails under 0.05 volt (11.2 kPa). The 1999 Ford PCED fails a frequency BARO PID and wants reference voltage greater than 4.0 volts.
- 2.Key on, engine off, record the barometric PID and a local station pressure in the same units. Do not apply a sea-level assumption or a per-thousand-feet rule as the failure line. The station reading is the comparison. The chart's minimum is the fail. The GRP09 74 kPa figure is an evaporative-monitor enable, not this code.
- 3.Read the diagram. If BARO is a separate sensor, as in the 1999 Ford PCED, test that sensor's reference, return, and signal. If BARO is captured from the MAP sensor at key-on, testing the MAP circuit is the barometric test, and agreeing MAP and BARO PIDs are one reading counted twice.
- 4.Check the MAP port and hose only when the diagram shows a hose and the chart says a pneumatic fault can set this plausibility code. A short to ground is the L61 and GRP09 description of P0107, and it is an electrical measurement: ignition off, sensor unplugged, signal wire to ground.
- 5.Measure a suspect signal wire end to end with the ignition off and both connectors disconnected. A voltage that matches at the sensor and at the module does not clear the wire. The input current is too small to show the drop across a corroded joint.
- 6.Do not use a wide-open-throttle pull on a public road to update the barometric value. If the chart really does update BARO at wide-open throttle, use the controlled procedure it prints, with the parameter recorded rather than watched by the driver.
- 7.Replace the sensor the diagram names after the reference, the signal, and the port have passed. Clear the code and compare key-on BARO with the station pressure again, then with the same minimum.
Repair cost
$100 – $450
A MAP or BARO sensor replacement runs about $100 to $450 in parts and labor. The low end is an accessible sensor and the high end is a Camry-class part.
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Related repairs
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.