Document ID: 2198193

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.

Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
Horn Switch Signal	1	3	3	—
Horn Switch Ground	—	3	—	—
Horn Relay Control	B2750 02	B2750 04	B2750 01	—
High Note Horn Control	3	2	1	—
High Note Horn Ground	—	2	—	—
Low Note Horn Control	3	2	1	—
High Low Horn Ground	—	2	—	—
1. Horn Always On 2. Single Horn Inoperative 3. Both Horns Inoperative

Circuit/System Description

The body control module (BCM) monitors the Horn Switch signal circuit, when the horn pad is pressed, the BCM detects the drop in the Horn Switch signal circuit and provides a ground for the Horn printed circuit board relay within the underhood fuse block. The underhood fuse block will then provide B+ voltage to the horn control circuit, sounding the horns.

Diagnostic Aids

OnStar® RemoteLink™ applications or "apps" downloads are available to owners of certain mobile devices. Unwanted or inadvertent horn activations have been associated with these applications to which the customer may be unaware. Contact the GM Technical Assistance Center regarding information on a remedy for this situation.

A short to ground in the horn relay control circuit or a short to voltage in the horn control circuit will cause the horns to sound continuously until the horns overheat and become inoperative.

Rotate the steering wheel while pressing the horn pad to identify intermittent and poor connections within the steering column.

If diagnosing a Horn- Poor Tone condition, inspect the following:

Debris or water in the horn assembly
Proper horn mounting hardware torque—Refer to Fastener Specifications.
Debris in the joint where the horns attach to the vehicle
Debris in direct contact with the horn
Vehicle components vibrating while the horn is sounding

Reference Information

Schematic Reference

Horn Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Horns System Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

Ignition ON.
Verify the scan tool Horn Switch parameter changes between Active and Inactive when pressing and releasing the horn pad.

If the parameter does not change
Refer to Circuit/System Testing — Horn Switch Malfunction.

If the parameter changes

Verify the P12A Horn-High Note and the P12B Horn-Low Note turns ON and OFF and emit a clear and even tone when commanding the Horn Relay On and OFF with a scan tool.

If the P12A Horn-High Note and the P12B Horn-Low Note do not turn ON and OFF
Refer to Circuit/System Testing – Horns or Horn Command Malfunction.

If a single P12 Horn does not turn ON and OFF
Refer to Circuit/System Testing – Horn - Poor Tone.

If the sound emitted is not clear and even
Refer to Circuit/System Testing – Horn - Poor Tone.

If the P12A Horn-High Note and the P12B Horn-Low Note turn ON and OFF and emit a even and clear tone

All OK.

Circuit/System Testing

Horn Switch Malfunction

Ignition OFF and all vehicle systems OFF, disconnect the X1 and X2 harness connectors at the S33 Horn Switch. It may take up to 2 minutes for all vehicle systems to power down.
Test for less than 10 Ω between the ground circuit terminal 1 X2 and ground.

If 10 Ω or greater

Ignition OFF.
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

Ignition ON.
Verify the scan tool Horn Switch parameter is Inactive.

If not Inactive

Ignition OFF, disconnect the harness connector at the K9 Body Control Module.
Test for infinite resistance between the signal circuit terminal 1 X1 and ground.

If less than infinite resistance, repair the short to ground on the circuit.
If infinite resistance, replace the K9 Body Control Module.

If Inactive

Install a 3 A fused jumper wire between the signal circuit terminal 1 X1 and the ground circuit terminal 1 X2.
Verify the scan tool Horn Switch parameter is Active.

If not Active

Ignition OFF, removed the 3 A fused jumper wire, disconnect the harness connector at the K9 Body Control Module, ignition ON.
Test for less than 1 V between the signal circuit and ground.

If 1 V or greater, repair the short to voltage on the circuit.

If less than 1 V

Test for less than 2 Ω in the signal 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 Active

Test or replace the S33 Horn Switch.

Horns or Horn Command Malfunction

Ignition OFF, disconnect the harness connector at the appropriate P12 Horn. Ignition ON
Connect a test lamp between the control circuit terminal A and ground, ignition ON.
Verify the test lamp does not turn ON and OFF when commanding the Horn Relay On and Off with a scan tool.

If the test lamp turns ON and OFF
Test or replace the P12 Horn.

If the test lamp does not turn ON and OFF

Ignition OFF, connect the harness connector at the P12 Horn. Disconnect the X1 harness connector at the X50A Fuse Block-Underhood, ignition ON.
Verify the P12A Horn-High Note and P12B Horn-Low Note are not activated.

If the P12A Horn-High Note and P12B Horn-Low Note is activated

Ignition OFF, disconnect the harness connector at the P12A Horn-High Note and P12B Horn-Low Note, ignition ON.
Test for less than 1 V between the control circuit terminal A and ground.

If 1 V or greater, repair the short to voltage on the circuit.
If less than 1 V, test or replace the P12A Horn-High Note and P12B Horn-Low Note.

If the P12A Horn-High Note and P12B Horn-Low Note is not activated

Connect a 15 A fused jumper wire between the control circuit terminal 32 and B+.
Verify the P12A Horn-High Note or P12B Horn-Low Note activates.

If the P12A Horn-High Note and P12B Horn-Low Note does not activate

Ignition OFF, remove the jumper wire, disconnect the harness connector at the P12 Horn, ignition ON.
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

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, test or replace the P12 Horn.

If the P12A Horn-High Note and P12B Horn-Low Note activates

Ignition OFF, connect the X1 harness connector at the X50A Fuse Block-Underhood. Disconnect the X5 harness connector at the K9 Body Control Module, ignition ON.
Verify the P12A Horn-High Note and P12B Horn-Low Note are not activated.

If the P12A Horn-High Note and P12B Horn-Low Note is activated

Ignition OFF, disconnect the X2 harness connector at the X50A Fuse Block-Underhood.
Test for infinite resistance between the control circuit terminal 19 X5 at the K9 Body Control Module and ground.

If less than Infinite resistance, repair the short to ground on the circuit.
If infinite resistance, replace the X50A Fuse Block-Underhood.

If the P12A Horn-High Note and P12B Horn-Low Note are not activated

Connect a 3 A fused jumper wire between the control circuit terminal 19 and ground.
Verify the P12A Horn-High Note and P12B Horn-Low Note activates.

If the P12 Horn does not activate

Ignition OFF, remove the jumper wire, disconnect the X2 harness connector at the X50A Fuse Block-Underhood, ignition ON.
Test for less than 1 V between the control circuit terminal 19 X5 at the K9 Body Control Module and ground.

If 1 V or greater, repair the short to voltage on the circuit.

If less than 1 V

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 X50A Fuse Block-Underhood.

If the P12A Horn-High Note and P12B Horn-Low Note activates

Replace the K9 Body Control Module.

Horn - Poor Tone

Ignition OFF and all vehicle systems OFF, disconnect the harness connector at the appropriate P12 Horn. It may take up to 2 minutes for all vehicle systems to power down.
Test for less than 1 Ω between the ground circuit terminal B and ground.

If 1 Ω or greater

Ignition OFF.
Test for less than 2 Ω in the ground circuit end to end.

If 2 Ω or greater, repair the high resistance in the circuit.
If less than 2 Ω, repair high resistance in the ground connection.

If less than 1 Ω

Ignition OFF, disconnect the X1 harness connector at the X50A Fuse Block-Underhood.
Test for less than 2 Ω between the P12 Horn control circuit terminal A and the X50A Fuse Block-Underhood control circuit terminal 32 X1.

If 2 Ω or greater
Repair the high resistance in the control circuit.

If less than 2 Ω

Test or replace the P12 Horn.

Component Testing

Horn Test

Ignition OFF, disconnect the harness connector at the appropriate P12 Horn.
Install a 15 A fused jumper wire between the control terminal A and 12 V. Install a jumper wire between the ground terminal B and ground.
Verify the P12 horn. turns ON and OFF

If the P12 Horn. does not turn ON and OFF
Replace the P12 Horn.

If the P12 Horn. turns ON and OFF

All OK

Horn Switch

Ignition OFF, disconnect the X1 and X2 harness connectors at the S33 Horn Switch.
Test for infinite resistance between the signal terminal 1 X1 and the ground terminal 1 X2 with the switch in the open position.

If less than infinite resistance
Replace the S33 Horn Switch.

If infinite resistance

Test for less than 3 Ω between the signal terminal 1 X1 and the ground terminal 1 X2 with the switch in the closed position.

If 3 Ω or greater
Replace the S33 Horn Switch.

If less than 3 Ω

All OK

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

Steering Wheel Horn Contact Replacement
Horn Replacement
Accessory Wiring Junction Block Replacement
Control Module References for body control module replacement, programming and setup

Document ID: 1875165

Important: The following steps must be completed before using the symptom tables:

Perform Diagnostic System Check - Vehicle , before using the symptom tables in order to verify that all of the following are true:

There are no DTCs set
The control modules can communicate via the serial data link

Review the system operation in order to familiarize yourself with the system functions. Refer to Horns System Description and Operation.

Visual/Physical Inspection

Inspect for aftermarket devices which could affect the operation of the horn system.
Inspect the easily accessible or visible system components for obvious damage or conditions which could cause the symptom.
Perform the following if a horn buzzes or has a harsh tone.

Inspect for debris in the joint where the horn fastens to the vehicle.
Test the torque of the horn mounting hardware. The horn mounting hardware should be tightened to a torque of 10 Nm (7 lb ft).

Intermittent

Faulty electrical connections or wiring may be the cause of intermittent conditions. Refer to Testing for Intermittent Conditions and Poor Connections.

Symptom List

Refer to a symptom diagnostic procedure Horns Malfunction in order to diagnose the symptom.

Item	Variant	Part #	Price
Horn · High Note	Horn	92228137	$0.00
Horn · Low Note	Horn	92228138	$0.00

Horn

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