AutoCodes – GMC – P21CB GMC

P21CB GMC Code - Reductant Control Module Supply Voltage Low Voltage

Quick Answer

The selective catalytic reduction system reduces Oxides of Nitrogen (NOx) emissions by injecting a metered amount of reductant into the exhaust gas stream entering the selective catalytic reduction catalyst. Common causes include Faulty Reductant Control Module, Reductant Control Module harness is open or shorted, Reductant Control Module 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: P21CB GMC indicates Reductant Control Module Supply Voltage Low Voltage.
Common causes: Common causes include Faulty Reductant Control Module, Reductant Control Module harness is open or shorted, Reductant Control Module 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 617

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Jul 27, 2021

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code P21CB GMC Description

The selective catalytic reduction system reduces Oxides of Nitrogen (NOx) emissions by injecting a metered amount of reductant into the exhaust gas stream entering the selective catalytic reduction catalyst. Within the selective catalytic reduction catalyst, the reductant reacts with the NOx to form nitrogen, carbon dioxide, and water vapor. Upstream and downstream NOx sensors provide the Engine Control Module (ECM) and the Reductant Control Module via serial data. The ECM controls the reductant injector duty cycle in response to changes in engine-out NOx levels. The Reductant Control Module supplies system voltage to the reductant pump motor. The Reductant Control Module controls the solenoid by grounding the control circuit. The Reductant Control Module can determine if the control circuit is open, shorted to ground, or shorted to a voltage by monitoring the feedback voltage.
The Diagnostic Trouble Code (DTC) will be set when the Reductant Control Module supply voltage is less than a calibrated value for greater than 5 s.

OBDII Code P21CB GMC - Reductant Control Module Supply Voltage Low Voltage - AutoCodes.com — P21CB GMC Code - Reductant Control Module Supply Voltage Low Voltage

What does this mean?

What are the Possible Causes of the DTC P21CB GMC?

Faulty Reductant Control Module
Reductant Control Module harness is open or shorted
Reductant Control Module circuit poor electrical connection

How to Fix the DTC P21CB 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 P21CB 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 P21CB GMC Code

1. What are the common symptoms of OBDII code P21CB GMC?

- Common symptoms include poor engine performance, reduced fuel efficiency, illuminated Check Engine Light, and potential engine stalling.

2. What are the possible causes of OBDII code P21CB GMC?

- The code P21CB in a GMC vehicle is often caused by a faulty reductant control module, damaged wiring or connectors, low battery voltage, or issues with the powertrain control module (PCM).

3. How can I diagnose the OBDII code P21CB in my GMC vehicle?

- Diagnosis usually involves using a professional OBDII scanner to read the fault codes stored in the vehicle's computer system. Additionally, a thorough visual inspection of wiring and connectors related to the reductant control module is recommended.

4. Can I continue driving my GMC vehicle with the P21CB code present?

- It is not recommended to drive your vehicle with the P21CB code present, as it can lead to further damage to the engine or emission control system. It's advisable to address the issue promptly.

5. How can I repair OBDII code P21CB in my GMC vehicle?

- Repairing the P21CB code may involve replacing the reductant control module, repairing or replacing damaged wiring or connectors, or addressing any issues with the power supply to the module.

6. Are there any DIY tips for fixing the P21CB code in my GMC vehicle?

- While some basic troubleshooting like checking for loose connections or damaged wiring can be done by DIY enthusiasts, it is recommended to consult a professional mechanic for a thorough diagnosis and repair of the issue.

7. How much does it typically cost to fix OBDII code P21CB in a GMC vehicle?

- The cost of repairing the P21CB code can vary depending on the specific cause of the issue, labor rates in your area, and the price of replacement parts. It's advisable to get a few quotes from different repair shops.

8. Can a low battery voltage trigger the P21CB code in a GMC vehicle?

- Yes, low battery voltage can be a potential cause of the P21CB code. It is important to ensure the vehicle's battery is in good condition and properly charged to prevent voltage-related issues.

9. Will resetting the Check Engine Light make the P21CB code disappear temporarily?

- Resetting the Check Engine Light may temporarily clear the code, but if the underlying issue causing the P21CB code is not addressed, the code is likely to reappear once the vehicle's computer system detects the problem again.

10. Is it necessary to take my GMC vehicle to a dealership for diagnosis and repair of the P21CB code?

- While dealerships have specialized knowledge of GMC vehicles, independent mechanics with experience in diagnosing and repairing OBDII codes can also effectively address the P21CB code. It's important to choose a reputable and qualified professional for the job.