AutoCodes – Generic – P21DD

P21DD 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 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 5,488

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

What does this mean?

What are the Possible Causes of the DTC P21DD?

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?

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 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.

Need more information about the P21DD code?

Need more information on how to fix the P21DD code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P21DD code diagnosed by a professional: Find a repair shop in your area

Information for Specific Makes

P21DD Chevrolet — Reductant Heater 1 Current Too Low
P21DD GMC — Reductant Heater 1 Current Too Low

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 Code

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

The common symptoms of this code may include the check engine light coming on, reduced engine performance, and potentially failing an emissions test.

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

Possible causes of this code include a faulty reductant heater, issues with the reductant heater circuit, a damaged wiring harness, or a problem with the powertrain control module (PCM).

3. How can I diagnose the cause of OBDII code P21DD?

Diagnosing this code usually involves using a scan tool to read the code and then conducting further testing to determine if the reductant heater is functioning properly, checking the wiring for any damage, and testing the PCM.

4. How serious is OBDII code P21DD?

While this code may not cause immediate drivability issues, it can lead to increased emissions and potentially affect the overall performance of the vehicle if left unaddressed.

5. Can I continue driving with OBDII code P21DD?

It is not recommended to ignore this code and continue driving, as it can lead to decreased fuel efficiency and potentially more serious issues over time. It's best to address the problem as soon as possible.

6. How can I repair OBDII code P21DD?

Repairing this code typically involves replacing the faulty reductant heater, repairing any damaged wiring, or addressing any other underlying issues that may be causing the fault.

7. Can I reset OBDII code P21DD without fixing the underlying issue?

While you can clear the code with a scan tool, it is important to address the underlying problem causing the fault to prevent the code from reoccurring.

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

The cost of repairing this code can vary depending on the specific cause of the issue, the labor rates in your area, and whether any parts need to be replaced. It's best to consult with a mechanic for an accurate estimate.

9. Does OBDII code P21DD affect vehicle safety?

This code typically does not impact the safety of the vehicle directly, but it is important to address it to ensure optimal vehicle performance and emissions compliance.

10. Can I prevent OBDII code P21DD from occurring?

While some causes of this code may be unavoidable, regular maintenance and prompt attention to any warning lights or unusual symptoms can help prevent issues that trigger this fault code.