AutoCodes – GMC – P21DD GMC

P21DD GMC Code - Reductant Heater 1 Current Too Low

Quick Answer

The Selective Catalyst Reduction (SCR) system reduces Oxides of Nitrogen (NOx) emissions by injecting a metered amount of Diesel Exhaust Fluid (DEF) or reductant into the exhaust gas stream entering the SCR catalyst. Common causes include Faulty Reductant Heater 1, Reductant Heater 1 harness is open or shorted, Reductant Heater 1 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: P21DD GMC indicates Reductant Heater 1 Current Too Low.
Common causes: Common causes include Faulty Reductant Heater 1, Reductant Heater 1 harness is open or shorted, Reductant Heater 1 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 3,991

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 Sep 10, 2021

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code P21DD GMC Description

The Selective Catalyst Reduction (SCR) system reduces Oxides of Nitrogen (NOx) emissions by injecting a metered amount of Diesel Exhaust Fluid (DEF) or reductant into the exhaust gas stream entering the SCR catalyst. Within the SCR 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) with engine-out and tailpipe-out NOx levels in Parts Per Million (ppm). The ECM then adjusts the amount of reductant injected upstream of the SCR based on the NOx concentration. Engine-out NOx levels as measured by NOx Sensor 1 will either increase (high ppm) or decrease (low ppm) the amount of reductant injected. The NOx concentration as measured by NOx Sensor 2 provides the ECM with both an indication of the NOx conversion efficiency within the SCR as well as confirming the proper operation of the reductant system. The Diagnostic Trouble Code (DTC) will be set when the Glow Plug Control Module (GPCM) detects a heater open circuit by detecting a low current in the heater.

OBDII Code P21DD GMC - Reductant Heater 1 Current Too Low - AutoCodes.com — P21DD GMC Code - Reductant Heater 1 Current Too Low

What does this mean?

What are the Possible Causes of the DTC P21DD GMC?

Faulty Reductant Heater 1
Reductant Heater 1 harness is open or shorted
Reductant Heater 1 circuit poor electrical connection
Faulty Glow Plug Control Module (GPCM)

How to Fix the DTC P21DD 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 P21DD 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 P21DD GMC Code

1. What are the common symptoms of OBDII code P21DD GMC (Reductant Heater 1 Current Too Low)?

The common symptoms include the check engine light illuminating on the dashboard, reduced engine performance, and potential issues with the vehicle's emissions system.

2. What are the possible causes of OBDII code P21DD GMC?

The code P21DD GMC typically indicates a malfunction in the Reductant Heater 1 circuit. Common causes include a faulty reductant heater 1, damaged wiring or connectors, or a problem with the reductant heater control module.

3. How serious is OBDII code P21DD GMC (Reductant Heater 1 Current Too Low)?

While the code itself may not cause immediate harm to the vehicle, it can lead to increased emissions and potential failure to pass emissions testing. It is recommended to address this issue promptly to prevent further complications.

4. How can I diagnose OBDII code P21DD GMC?

Diagnosing this code typically involves using an OBDII scanner to retrieve the specific fault code, conducting a visual inspection of the reductant heater 1 components, and testing the wiring and connectors for continuity and proper voltage.

5. Can I still drive my vehicle with OBDII code P21DD GMC?

It is generally safe to drive the vehicle with this code, but it is recommended to address the issue as soon as possible to prevent any potential damage to the emissions system and ensure optimal vehicle performance.

6. How can I repair OBDII code P21DD GMC?

Repairing this code may involve replacing the faulty reductant heater 1, repairing any damaged wiring or connectors, or resetting the reductant heater control module. It is advisable to consult a professional mechanic for an accurate diagnosis and repair.

7. Will OBDII code P21DD GMC cause my vehicle to fail emissions testing?

Yes, vehicles with unresolved emissions-related issues like OBDII code P21DD GMC are likely to fail emissions testing. It is important to address the problem to ensure compliance with emissions regulations.

8. How much does it cost to repair OBDII code P21DD GMC?

The cost of repairing this code can vary depending on the extent of the issue, the parts needed for replacement, and labor costs. It is recommended to obtain quotes from different mechanics or repair shops for an accurate estimate.

9. Can a DIY enthusiast fix OBDII code P21DD GMC at home?

While some experienced DIY enthusiasts may be able to diagnose and repair this code at home, it is recommended to consult a professional mechanic for accurate diagnosis and repair to ensure the problem is properly addressed.

10. What happens if I ignore OBDII code P21DD GMC?

Ignoring this code can lead to increased emissions, potential damage to the emissions system, and reduced fuel efficiency. It is advisable to address the issue promptly to prevent further complications.