For 1990-2009 cars

### Circuit Description

The brake lamp switch is a normally open switch that closes when the brake pedal is depressed. When the brake pedal is depressed, the brake lamp switch supplies signal voltage to the brake lamp switch signal circuit. The brake pedal switch is a normally closed switch that opens when the brake pedal is depressed. The engine control module (ECM) monitors the brake lamp switch signal circuit and the brake pedal switch signal circuit. The ECM monitors the status of the brake lamp switch signal circuit and the brake pedal switch signal circuit to see if they match when the brake pedal is pressed or released.

### Conditions for Running the DTC

The ignition is ON.

### Conditions for Setting the DTC

• | The engine is running.

• | If the time from state change of one brake input to when the other brake input changes state (making switch states equal again) is greater than 0.5 second, fail counts increase by 3. When the fail counts reach 39, this DTC sets.

### Action Taken When the DTC Sets

When DTC P0504 is set the following will occur:

• | The malfunction indicator lamp (MIL) will illuminate.

• | The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.

• | A history DTC is stored

### Conditions for Clearing the DTC

• | The MIL will turn off after 4 consecutive ignition cycles in which the diagnostic runs without a fault.

• | A history DTC will clear after 40 consecutive warm-up cycles without a fault.

• | DTC(s) can be cleared by using the scan tool.

• | Disconnecting the ECM battery feed for more than 10 seconds

### Diagnostic Aids

• | Inspect for proper adjustment of the stop lamp switch. Refer to Stop Lamp Switch Adjustment .

• | Check for intermittent and poor connections. Refer to Testing for Intermittent Conditions and Poor Connections .

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Exterior Lights Schematics and Cruise Control Schematics Connector End View Reference: Component Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle .
2 |

1. With the scan tool, observe the brake lamp switch and the brake pedal switch parameter.
2. Press and release the brake pedal.

Does the Brake Pedal Switch parameter display change with the brake pedal position?

Go to Diagnostic Aids

Go to Step 3

3 |

1. With the scan tool, observe the brake pedal switch parameter.
2. Press and release the brake pedal.

Does the brake pedal switch parameter display change with the brake pedal position?

Go to Step 6

Go to Step 4

4 |

1. Turn OFF the ignition.
2. Disconnect the brake lamp switch
3. Turn ON the ignition, with the engine OFF.

Does the brake pedal; switch parameter display released?

Go to Step 5

Go to Step 12

5 |

1. Turn OFF the ignition.
2. Connect a 10-amp fused jumper between the ignition positive voltage circuit of the brake lamp switch and the brake pedal switch signal circuit of the brake lamp switch.
3. Turn ON the ignition, with the engine OFF.

Does the brake pedal switch parameter display applied?

Go to Step 15

Go to Step 8

6 |

1. Turn OFF the ignition.
2. Disconnect the brake lamp switch.
3. Turn ON the ignition, with the engine OFF.
4. With the scan tool, observe the brake lamp switch parameter.

Does the brake lamp switch parameter display released?

Go to Step 7

Go to Step 10

7 |

1. Turn OFF the ignition.
2. Connect a 20-amp fused jumper between the battery positive voltage circuit of the brake lamp switch and the brake lamp signal circuit
3. Turn ON the ignition, with the engine OFF.

Does the brake lamp switch parameter display applied?

Go to Step 15

Go to Step 11

8 | Test the ignition positive voltage circuit of the brake lamp switch for an open or short to ground. Did you find and correct the condition? | Go to Step 18 | Go to Step 9
9 | Test for an open or short to ground in the brake pedal signal circuit of the brake lamp switch. Did you find and correct the condition? | Go to Step 18 | Go to Step 14
10 | Test the brake lamp switch signal circuit for a short to voltage. Did you find and correct the condition? | Go to Step 18 | Go to Step 14
11 | Test the battery positive voltage circuit of the brake lamp switch for an open or short to ground. Did you find and correct the condition? | Go to Step 18 | Go to Step 13
12 | Test the brake pedal switch signal circuit of the brake lamp switch for a short to voltage. Did you find and correct the condition? | Go to Step 18 | Go to Step 16
13 | Test the brake lamp switch signal circuit for an open or short to ground. Did you find and correct the condition? | Go to Step 18 | Go to Step 14
14 | Inspect for poor connections at the harness connector of the engine control module (ECM). Did you find and correct the condition? | Go to Step 18 | Go to Step 16
15 | Inspect for poor connections at the harness connector of the brake lamp switch. Did you find and correct the condition? | Go to Step 18 | Go to Step 17
16 | Replace the ECM. Refer to Control Module References. Did you complete the replacement? | Go to Step 18 | --
17 | Replace the brake lamp switch. Did you complete the replacement? | Go to Step 18 | --
18 |

1. Use the scan tool in order to clear the DTCs.
2. Operate the vehicle within the conditions for setting the DTC.

Does the DTC reset?

Go to Step 2

System OK