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DTC B388A

DTC B388A

Diagnostic Instructions


• 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.


DTC Descriptor

DTC B388A 01

Trailer Park Lamps Circuit Short to Battery

DTC B388A 02

Trailer Park Lamps Circuit Short to Ground

DTC B388A 04

Trailer Park Lamps Circuit Open

Diagnostic Fault Information

Circuit Short to Ground Open/High Resistance Short to Voltage Signal Performance
Trailer Park Lamp Relay B+ B388A 04 B388A 04 - -
Park Lamp Supply Voltage/Marker Lamp Supply Voltage B388A 02 B388A 04 B388A 01 -


Circuit/System Description

The trailer park lamp relay is supplied with battery voltage at all times. The trailer park lamp signal circuit is grounded by turning the headlamp switch. The body control module (BCM) energizes the trailer park lamp relay by applying ground to the trailer park lamp relay control circuit. When the trailer park lamp relay is energized, the relay switch contacts close and battery voltage is applied through the trailer park lamp relay fuse to the trailer park lamp supply voltage circuit which illuminates the trailer park lamps if a trailer is connected.

Conditions for Running the DTC

When the trailer park lamp relay is actively being requested by the BCM.

Conditions for Setting the DTC

B388A 01

The BCM detects a short to voltage on the park lamp supply voltage/marker lamp supply voltage circuit.

B388A 02

The BCM detects a short to ground on the park lamp supply voltage/marker lamp supply voltage circuit

B388A 04

The BCM detects an open/high resistance on the park lamp supply voltage/marker lamp supply voltage circuit

Action Taken When the DTC Sets

B388A 01 or B388A 04

The trailer park lamps are inoperative

B388A 02

The trailer park lamps are always ON

Conditions for Clearing the DTC


• The DTC will be current for as long as the fault is present.

• When the fault is no longer present, the DTC will be a history DTC.

• A history DTC will clear after 40 ignition cycles.


Reference Information

Schematic Reference

Trailer Connector/Provision Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Exterior Lighting Systems Description and Operation

Electrical Information Reference


• Circuit Testing

• Connector Repairs

• Testing for Intermittent Conditions and Poor Connections

• Wiring Repairs


Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

Ignition ON, observe the function of the trailer tail lamps while commanding the Trailer Park Lamps ON and OFF with a scan tool. The trailer tail lamps should turn ON and OFF while changing between the commanded states.

Circuit/System Testing


1. Ignition OFF, disconnect the trailer park lamp relay.

2. Ignition ON, verify a test lamp illuminates between the B+ circuit terminal 86 and ground.

3. If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

4. Connect a test lamp between the control circuit terminal 85 and the B+ circuit terminal 86.

5. Command the Trailer Park Lamps ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

6. If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the K9 BCM.

7. If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the K9 BCM

8. If all circuits test normal, test or replace the trailer park lamp relay.


Component Testing


Relay Test

1. Ignition OFF, disconnect the trailer park lamp relay.

2. Test for 60-180 Ω between terminals 85 and 86.

3. If not within the specified range, replace the trailer park lamp relay.

4. Test for infinite resistance between the following terminals:

• 30 and 86

• 30 and 87

• 30 and 85

• 85 and 87

5. If not the specified value, replace the trailer park lamp relay.

6. Install a 5 A fused jumper wire between relay terminal 85 and 12 V. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 Ω between terminals 30 and 87.

7. If greater than the specified range, replace the trailer park lamp relay.


Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.


• Relay Replacement (Attached to Wire Harness) Relay Replacement (Within an Electrical Center)

• Control Module References for BCM replacement, programming and setup