AutoCodes – GMC – B3821 GMC

B3821 GMC Code - Left Rear Window Relay Power Circuit

Quick Answer

Battery positive voltage is supplied to the left rear power window switch and the up and down relays through the window lockout signal circuit. Common causes include Faulty Body Control Module (BCM), Body Control Module (BCM) harness is open or shorted, Body Control Module (BCM) circuit poor electrical connection. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: B3821 GMC indicates Left Rear Window Relay Power Circuit.
Common causes: Common causes include Faulty Body Control Module (BCM), Body Control Module (BCM) harness is open or shorted, Body Control Module (BCM) circuit poor electrical connection.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 427

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

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Table of contents Created and updated by William Orellana on Mar 17, 2020

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code B3821 GMC Description

Battery positive voltage is supplied to the left rear power window switch and the up and down relays through the window lockout signal circuit. If the window lockout function is disabled, the Body Control Module (BCM) supplies battery positive voltage to the switch and relays and the left rear window operates normally. If the window lockout function is enabled, the BCM will not supply battery positive voltage to the switch and relays and the left rear window will be inoperative unless it is operated from the driver power window switch. The Diagnostic Trouble Code (DTC) sets when the window lockout function is disabled and the window lockout signal circuit is shorted to ground.

OBDII Code B3821 GMC - Left Rear Window Relay Power Circuit - AutoCodes.com — B3821 GMC Code - Left Rear Window Relay Power Circuit

What does this mean?

What are the Possible Causes of the DTC B3821 GMC?

Faulty Body Control Module (BCM)
Body Control Module (BCM) harness is open or shorted
Body Control Module (BCM) circuit poor electrical connection

How to Fix the DTC B3821 GMC?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the B3821 GMC code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

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About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about B3821 GMC Code

1. What does the OBDII code B3821 mean on a GMC vehicle?

B3821 indicates an issue with the Left Rear Window Relay Power Circuit, which may involve a fault in the relay, wiring, or associated components.

2. What are the common symptoms of the B3821 code?

Symptoms may include the left rear window not operating, intermittent window functionality, or no response when using the window switch.

3. What causes the B3821 code to appear?

Common causes include a faulty window relay, damaged wiring or connectors in the circuit, a malfunctioning window switch, or a problem with the control module.

4. Can I still drive my GMC with the B3821 code active?

Yes, you can drive the vehicle, but the left rear window may not function properly. It's best to address the issue promptly to avoid further electrical problems.

5. How is the B3821 code diagnosed?

A technician will use a scan tool to confirm the code, inspect the wiring and connectors for damage, test the window relay, and check the switch and control module for proper operation.

6. Can a blown fuse cause the B3821 code?

Yes, a blown fuse in the window circuit may cause this code. Inspect and replace any blown fuses before performing further diagnostics.

7. How do you fix the B3821 code?

Repairs may involve replacing a faulty window relay, repairing or replacing damaged wiring, cleaning or reconnecting loose connectors, or replacing a defective switch or control module.

8. How much does it cost to fix the B3821 code?

The cost varies based on the cause. Replacing a window relay may cost $50-$150, while wiring or control module repairs can be more expensive.

9. Can I fix the B3821 code myself?

If you have experience with automotive electrical systems, you may be able to inspect and replace the relay or check the wiring. For more complex issues, professional assistance is recommended.

10. Will clearing the B3821 code with a scan tool fix the problem?

Clearing the code will not fix the underlying issue. If the problem persists, the code will return until the root cause is resolved.