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P21AA GMC Code - Reductant Level Sensor 2 Circuit Low Voltage

Quick Answer

The reductant level/temperature sensor is a solid-state device that provides the Engine Control Module (ECM) with information on reductant level and temperature. Common causes include Faulty Reductant Level Sensor 2, Reductant Level Sensor 2 harness is open or shorted, Reductant Level Sensor 2 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
P21AA GMC indicates Reductant Level Sensor 2 Circuit Low Voltage.
Common causes
Common causes include Faulty Reductant Level Sensor 2, Reductant Level Sensor 2 harness is open or shorted, Reductant Level Sensor 2 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 395
How popular is this code, among other codes? Low
What is this?
Table of contents Created and updated by William Orellana on Sep 10, 2021
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Code P21AA GMC Description

The reductant level/temperature sensor is a solid-state device that provides the Engine Control Module (ECM) with information on reductant level and temperature. The level/temperature sensor receives system voltage directly from the underhood electrical center and ground through a chassis ground circuit. The level sensor consists of three sensing elements. Reductant level sensor 3 is highest, reductant level sensor 2 is located between sensors 1 and 3. Reductant level sensor 1 is located at a low reductant level. Reductant level sensor 3 is located at less than half the range of the DEF tank level. The scan tool will display Active when the level sensors are in contact with the reductant and Inactive when not in contact with the reductant. An integrated temperature sensor monitors reductant temperature. Reductant level and temperature signals are processed within the sensor and supplied to the ECM as serial data messages. The ECM will set a DTC when it detects an invalid level or temperature serial data message from the sensor. The Diagnostic Trouble Code (DTC) will be set when the ECM receives a serial data message from the reductant sensor module that the emission reduction fluid tank level sensor 1, 2, or 3 voltage is less than 0.17 V for greater than 3 s. A voltage within this range indicates a short to ground.
OBDII Code P21AA GMC - Reductant Level Sensor 2 Circuit Low Voltage - AutoCodes.com
P21AA GMC Code - Reductant Level Sensor 2 Circuit Low Voltage
What does this mean?

What are the Possible Causes of the DTC P21AA GMC?

  • Faulty Reductant Level Sensor 2
  • Reductant Level Sensor 2 harness is open or shorted
  • Reductant Level Sensor 2 circuit poor electrical connection

How to Fix the DTC P21AA 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 P21AA 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 P21AA GMC Code

1. What does the OBDII code P21AA mean on a GMC?

P21AA indicates a "Reductant Level Sensor 2 Circuit Low Voltage" issue, meaning the vehicle's ECM has detected a problem with the electrical circuit for the second reductant level sensor.

2. What are the common symptoms of the P21AA code?

Symptoms may include the Check Engine Light or DEF (Diesel Exhaust Fluid) warning light, reduced engine performance, and possible warnings about the DEF system.

3. What causes the P21AA code to appear?

Common causes include a faulty reductant level sensor, damaged or corroded wiring, a poor electrical connection, or a defective reductant tank module.

4. Can I still drive my GMC with a P21AA code?

You may be able to drive temporarily, but ignoring the issue can lead to reduced engine power or the inability to restart the vehicle if the DEF system malfunctions.

5. How do I diagnose the P21AA code?

Start by scanning for related codes, inspecting the reductant level sensor wiring and connectors, and testing the voltage and continuity of the circuit with a multimeter.

6. How do I fix the P21AA code on my GMC?

Repairs may include replacing the reductant level sensor, repairing or replacing damaged wiring, cleaning corroded connections, or replacing the reductant tank assembly if necessary.

7. Is the reductant level sensor the same as the DEF sensor?

Yes, the reductant level sensor monitors the amount of DEF (Diesel Exhaust Fluid) in the tank and reports it to the vehicle's ECM.

8. How much does it cost to fix a P21AA code?

The repair costs can vary, ranging from $200 to $1,000 or more, depending on whether you need to replace the sensor, wiring, or the reductant tank assembly.

9. Can a low DEF level trigger the P21AA code?

No, the code specifically refers to an electrical issue with the sensor circuit, not the actual DEF fluid level.

10. What tools are needed to troubleshoot a P21AA code?

You’ll need an OBDII scanner, a multimeter to test electrical circuits, and possibly a wiring diagram for your specific GMC vehicle.

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