P0129 - Barometric Pressure Out-Of-Range Low: Notes

Fig 1: Identifying Manifold Absolute Pressure Sensor Wiring Diagram

Courtesy of VOLKSWAGEN GROUP OF AMERICA, INC.

Theory of Operation

The Barometric Pressure (BARO) Sensor is the Manifold Absolute Pressure (MAP) Sensor which is a transducer that varies resistance according to changes in altitude and atmospheric conditions. The BARO reading, which is read the first few moments before engine cranking, gives the Powertrain Control Module (PCM) an indication of the current barometric pressure. The PCM uses this information to calculate fuel delivery. The PCM supplies 5 Volts to the MAP Sensor on a 5-Volt reference circuit and provides a ground on a low reference circuit. The MAP sensor provides a voltage signal to the PCM on a signal circuit relative to the pressure changes. The PCM monitors the stability of the BARO reading output by comparing successive samples.

>>When Monitored:<< With the ignition key on. No Cam or Crank signal within 75 ms. Engine speed less than 250 RPM.
>>Set Condition:<< The Powertrain Control Module (PCM) senses the voltage from the MAP sensor to be less than 2.2 Volts but above 0.04 of a Volt for 300 milliseconds. One Trip Fault. Three good trips to turn off the MIL. MIL is illuminated and the ETC lamp will flash.

Possible Causes
(F856) 5-VOLT SUPPLY CIRCUIT SHORTED TO VOLTAGE
(F856) 5-VOLT SUPPLY CIRCUIT OPEN
(F856) 5-VOLT SUPPLY CIRCUIT SHORTED TO GROUND
(K1) MAP SIGNAL CIRCUIT OPEN
(K1) MAP SIGNAL CIRCUIT SHORTED TO GROUND
(K1) MAP SIGNAL CIRCUIT SHORTED TO THE (K900) SENSOR GROUND
MAP SENSOR
POWERTRAIN CONTROL MODULE (PCM)

Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Refer to Pre-Diagnostic Troubleshooting Procedure .