For 1990-2009 cars

### Circuit Description

The brake booster pressure sensor measures the negative air pressure or vacuum in the brake booster assembly. The engine control module (ECM) supplies the following circuits to the brake booster pressure sensor:

• | A 5-volt reference circuit

• | A signal circuit

• | A low reference circuit

During engine running conditions, the brake booster pressure sensor signal circuit voltage varies, depending on the vacuum reserve internal to the brake booster. If the ECM detects that the brake booster vacuum is less than the intake manifold vacuum, then DTC P0556 sets.

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0556 Brake Booster Pressure Sensor Performance

### Conditions for Running the DTC

• | The engine is running at an idle condition.

• | The ignition voltage is greater than 11 volts, but less than 18 volts.

• | DTC P0556 runs continuously once the above conditions are met.

### Conditions for Setting the DTC

The ECM detects brake booster vacuum as being less than the intake manifold vacuum for more than 100 milliseconds.

### Action Taken When the DTC Sets

• | The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.

• | The control module records the operating conditions at the time the diagnostic fails. The control module stores the information in the Freeze Frame/Failure Records.

### Conditions for Clearing the MIL/DTC

• | The control module turns OFF the MIL after 3 consecutive ignition cycles that the diagnostic runs and does not fail.

• | A current DTC, last test failed, clears when the diagnostic runs and passes.

• | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.

• | Clear the MIL and the DTC with a scan tool.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views and Engine Controls Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Are DTCs P258A, P258B, P258C, P258D, P0557, or P0558 also set? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 3
3 |

1. Observe the Freeze Frame/Failure Records for this DTC.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Does the DTC set during this ignition cycle?

--

Go to Step 4

Go to Testing for Intermittent Conditions and Poor Connections

4 |

1. Turn OFF the ignition.
2. Disconnect the brake booster pressure sensor electrical connector.
3. Test for shorted terminals and poor connections at the brake booster pressure sensor wire harness electrical connector, and also the mating electrical connector on the brake booster pressure sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

--

Go to Step 17

Go to Step 5

5 |

1. Remove the brake booster pressure sensor from the brake booster assembly.
2. Install a vacuum gage to the brake booster pressure sensor port on the brake booster assembly.
3. Start the engine.
4. Allow the engine to reach normal operating temperatures.
5. Monitor the vacuum calculated parameter in the cylinder deactivation data list, with a scan tool.

Is the difference between the vacuum gage and the vacuum calculated parameter more than the specified value?

7 kPa (2 in Hg)

Go to Brake Assist System Diagnosis

Go to Step 6

6 |

1. Turn OFF the ignition.
2. Connect a J 23738-A Mityvac to the vacuum port on the brake booster pressure sensor.
3. Apply vacuum pressure to the brake booster pressure sensor until 20 inches Hg is reached.
4. Monitor the vacuum gage on the Mityvac for 30 seconds.

Did the vacuum reading decrease after 30 seconds?

--

Go to Step 15

Go to Step 7

7 |

1. Connect the wire harness electrical connector to the brake booster pressure sensor.
2. Connect a J 23738-A to the vacuum port on the brake booster pressure sensor.
3. Turn ON the ignition, with the engine OFF.
4. Monitor the brake booster pressure sensor parameter on the scan tool.
5. Apply vacuum to the brake booster pressure sensor until 15 inches Hg is reached.

Is the difference between the Mityvac Gage display and the brake booster pressure sensor parameter more than the specified value?

7 kPa (2 in Hg)

Go to Step 8

Go to Testing for Intermittent Conditions and Poor Connections

8 |

1. Turn OFF the ignition.
2. Disconnect the engine control module (ECM) electrical connectors.
3. Test for shorted terminals and poor connections at the ECM wire harness electrical connectors and the mating electrical on the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

--

Go to Step 17

Go to Step 9

9 | Measure the resistance of the 5-volt reference circuit between the ECM electrical connector and the brake booster pressure sensor electrical connector. Refer to Circuit Testing . Does the resistance measure greater than the specified value? | 2 ohms | Go to Step 12 | Go to Step 10
10 | Measure the resistance of the signal circuit between the ECM electrical connector and the brake booster pressure sensor electrical connector. Refer to Circuit Testing . Does the resistance measure greater than the specified value? | 2 ohms | Go to Step 13 | Go to Step 11
11 | Measure the resistance of the low reference circuit between the ECM electrical connector and the brake booster pressure sensor electrical connector. Refer to Circuit Testing . Does the resistance measure greater than the specified value? | 2 ohms | Go to Step 14 | Go to Step 16
12 | Repair the high resistance in the 5-volt reference circuit of the brake booster pressure sensor. Refer to Wiring Repairs . Did you complete the repairs? | -- | Go to Step 17 | --
13 | Repair the high resistance in the signal circuit of the brake booster pressure sensor. Refer to Wiring Repairs . Did you complete the repairs? | -- | Go to Step 17 | --
14 | Repair the high resistance in the low reference circuit of the brake booster pressure sensor. Refer to Wiring Repairs . Did you complete the repairs? | -- | Go to Step 17 | --
15 | Replace the brake booster pressure sensor. Did you complete the replacement? | -- | Go to Step 17 | --
16 | Replace the ECM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 17 | --
17 |

1. Clear the DTCs with a scan tool.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 18

18 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle | System OK