• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
• Review Strategy Based Diagnosis for an overview of the diagnostic approach.
• Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Ground is applied at all times to the turn signal/multifunction switch. The turn signal lamps may only be activated with the ignition switch in the ON or START positions. When the turn signal/multifunction switch is placed in either the TURN RIGHT or TURN LEFT position, ground is applied to the body control module (BCM) through either the right turn or left turn signal switch signal circuit. The BCM responds to the turn signal switch input by applying a pulsating voltage to the front and rear turn signal lamps through there respective control circuits. When a turn signal request is received by the BCM, a serial data message is sent to the instrument cluster requesting the respective turn signal indicator be pulsed ON and OFF.

• X88 = Chevrolet
• Z88 = GMC

Battery voltage must be between 9-16 V.

• The BCM detects an open/high resistance in the left rear turn signal control circuit.
• The BCM detects an open/high resistance in the left rear turn signal ground circuit.

The BCM detects a short to ground in the left rear turn signal control circuit.

• The BCM detects an open/high resistance in the right rear turn signal control circuit.
• The BCM detects an open/high resistance in the right rear turn signal ground circuit.

The BCM detects a short to ground in the right rear turn signal control circuit.

• The left rear turn signal is inoperative.
• The “Check Left Rear Turn Signal Lamp” message is displayed on the driver information center display.
• The left turn signal indicator on the instrument cluster flashes rapidly.

• The right rear turn signal is inoperative.
• The “Check Right Rear Turn Signal Lamp” message is displayed on the driver information center display.
• The right turn signal indicator on the instrument cluster flashes rapidly.

• The condition responsible for setting the DTC no longer exists.
• A history DTC will clear once 100 consecutive malfunction-free ignition cycles have occurred.

When testing at the rear turn signal lamp bulb socket, make sure all measurements and test lamp connections are in the correct terminal location and do not come in contact with each other during testing.

Exterior Lights Schematics

Master Electrical Component List

Exterior Lighting Systems Description and Operation

• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs

Control Module References for scan tool information

1. Ignition OFF, exterior lamps OFF, disconnect the harness connector at the appropriate component listed below.
• {X88 without UGE} E5LE Tail/Stop Lamp – Left Outer
• {X88 with UGE} E42L Tail Lamp Assembly – Left
• {Z88} E5LE Tail/Stop Lamp – Left Outer
2. Test for less than 10 Ω between the appropriate ground circuit terminal listed below and ground.
• {X88 without UGE} E5LE Tail/Stop Lamp – Left Outer socket terminal
• {X88 with UGE} E42L Tail Lamp Assembly – Left terminal 4
• {Z88} E5LE Tail/Stop Lamp – Left Outer socket terminal
• If 10 Ω or greater
1. Ignition OFF.
2. Test for less than 2 Ω in the ground circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, repair the open/high resistance in the ground connection.
• If less than 10 Ω
3. Connect a test lamp between the appropriate control circuit terminal listed below and ground, ignition ON.
• {X88 without UGE} E5LE Tail/Stop Lamp – Left Outer socket terminal
• {X88 with UGE} E42L Tail Lamp Assembly – Left terminal 1
• {Z88} E5LE Tail/Stop Lamp – Left Outer socket terminal
4. Verify the test lamp turns ON and OFF when commanding the Left Rear Turn Signal Lamp ON and OFF with a scan tool.
• If the test lamp is always OFF
1. Ignition OFF, disconnect the X4 harness connector at the K9 Body Control Module.
2. Test for infinite resistance between the control circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 Ω in the control circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, replace the K9 Body Control Module.
• If the test lamp is always ON
1. Ignition OFF, disconnect the X4 harness connector at the K9 Body Control Module, ignition ON.
2. Test for less than 1 V between the control circuit terminal and ground.
• If 1 V or greater, repair the short to voltage on the circuit.
• If less than 1 V, replace the K9 Body Control Module.
• If the test lamp turns ON and OFF
5. Test or replace the E5LE Tail/Stop Lamp – Left Outer.

1. Ignition OFF, exterior lamps OFF, disconnect the harness connector at the appropriate component listed below.
• {X88 without UGE} E5RE Tail/Stop Lamp – Right Outer
• {X88 with UGE}E42R Tail Lamp Assembly – Right
• {Z88} E5RE Tail/Stop Lamp – Right Outer
2. Test for less than 10 Ω between the appropriate ground circuit terminal listed below and ground.
• {X88 without UGE} E5RE Tail/Stop Lamp – Right Outer socket terminal
• {X88 with UGE} E42R Tail Lamp Assembly – Right terminal 4
• {Z88} E5RE Tail/Stop Lamp – Right Outer socket terminal
• If 10 Ω or greater
1. Ignition OFF.
2. Test for less than 2 Ω in the ground circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, repair the open/high resistance in the ground connection.
• If less than 10 Ω
3. Connect a test lamp between the appropriate control circuit terminal listed below and ground, ignition ON.
• {X88 without UGE} E5RE Tail/Stop Lamp – Right Outer socket terminal
• {X88 with UGE} E42R Tail Lamp Assembly – Right terminal 1
• {Z88} E5RE Tail/Stop Lamp – Right Outer socket terminal
4. Verify the test lamp turns ON and OFF when commanding the Right Rear Turn Signal Lamp ON and OFF with a scan tool.
• If the test lamp is always OFF
1. Ignition OFF, disconnect the X5 harness connector at the K9 Body Control Module.
2. Test for infinite resistance between the control circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 Ω in the control circuit end to end.
• If 2 Ω or greater, repair the open/high resistance in the circuit.
• If less than 2 Ω, replace the K9 Body Control Module.
• If the test lamp is always ON
1. Ignition OFF, disconnect the X5 harness connector at the K9 Body Control Module, ignition ON.
2. Test for less than 1 V between the control circuit terminal and ground.
• If 1 V or greater, repair the short to voltage on the circuit.
• If less than 1 V, replace the K9 Body Control Module.
• If the test lamp turns ON and OFF
5. Test or replace the E5RE Tail/Stop Lamp – Right Outer.

Perform the Diagnostic Repair Verification after completing the repair.

• Rear Combination Lamp Replacement
• Control Module References for BCM replacement, programming, and setup