2014 Jeep Wrangler JK Factory Service Manual 28 - DTC-Based Diagnostics

P0129-BAROMETRIC PRESSURE OUT-OF-RANGE LOW

For a complete wiring diagram, refer to the Wiring Information.


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.


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 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).


1. ACTIVE DIAGNOSTIC TROUBLE CODE (DTC)

1. Ignition on, engine not running.
2. With the scan tool, read the active DTCs.


Is the DTC Active at this time.

Yes


No


2. (F856) 5-VOLT SUPPLY CIRCUIT
1. Turn the ignition off.
2. Disconnect the MAP Sensor harness connector.
3. Ignition on, engine not running.
4. Measure the voltage on the (F856) 5-Volt Supply circuit in the MAP Sensor harness connector.


Is the voltage between 4.5 to 5.2 Volts?

Yes


No


3. MAP SENSOR

1. With the scan tool, monitor the MAP Sensor voltage with the Sensor harness connector disconnected.


Is the voltage above 2.2 Volts?

Yes


No


4. (K1) MAP SIGNAL CIRCUIT OPEN
1. Turn the ignition off.
2. Disconnect the PCM C2 harness connector.
3. Measure the resistance of the (K1) MAP Signal circuit from the MAP Sensor harness connector to the PCM C2 harness connector.


Is the resistance below 5.0 Ohms?

Yes


No


5. (K1) MAP SIGNAL CIRCUIT SHORTED TO THE (K900) SENSOR GROUND CIRCUIT
1. Measure the resistance between the (K900) Sensor ground circuit and the (K1) MAP Signal circuit in the MAP Sensor harness connector.


Is the resistance below 100 Ohms?

Yes


No


6. (K1) MAP SIGNAL CIRCUIT SHORTED TO GROUND
1. Measure the resistance between ground and the (K1) MAP Signal circuit in the MAP Sensor harness connector.


Is the resistance below 100 Ohms?

Yes


No


7. (F856) 5-VOLT SUPPLY CIRCUIT SHORTED TO VOLTAGE
1. Turn the ignition off.
2. Disconnect the PCM C2 harness connector.
3. Ignition on, engine not running.
4. Measure the voltage on the (F856) 5-Volt Supply circuit in the MAP Sensor harness connector.


Is there any voltage present?

Yes


No


8. (F856) 5-VOLT SUPPLY CIRCUIT OPEN
1. Turn the ignition off.
2. Measure the resistance of the (F856) 5-Volt Supply circuit from the MAP Sensor harness connector to the PCM C2 harness connector.


Is the resistance below 5.0 Ohms?

Yes


No


9. (F856) 5-VOLT SUPPLY CIRCUIT SHORTED TO GROUND
1. Measure the resistance between ground and the (F856) 5-Volt Supply circuit in the MAP Sensor harness connector.


Is the resistance below 100 Ohms?

Yes


No


10. POWERTRAIN CONTROL MODULE (PCM)

1. Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the MAP Sensor and the Powertrain Control Module (PCM).
2. Look for any chafed, pierced, pinched or partially broken wires.
3. Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact in the MAP Sensor and Powertrain Control Module connectors.
4. Perform any Technical Service Bulletins that may apply.


Were there any problems found?

Yes


No