Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
This code means that the car's computer has detected a problem with a sensor that checks how much special fluid is in the tank. This fluid helps clean the car's exhaust. It's like a fuel gauge for this special fluid, and the car thinks the gauge is not working right.
The P21AA code indicates that the reductant level sensor B circuit is reporting a low voltage, suggesting a potential issue with the sensor or its wiring.
The reductant level sensor is part of the vehicle's Selective Catalytic Reduction (SCR) system, which uses Diesel Exhaust Fluid (DEF) to reduce emissions. This sensor monitors the level of DEF in the tank. A 'circuit low' condition suggests that the sensor is not sending the expected voltage to the Engine Control Module (ECM), possibly due to a wiring issue, a faulty sensor, or a problem with the ECM itself.
• Check Engine Light illuminated
• Possible reduction in engine performance
• Warning message for DEF system
• Faulty reductant level sensor
• Damaged or corroded wiring/connectors
• Defective ECM
Reductant: A substance used to reduce emissions in the exhaust system.
SCR: Selective Catalytic Reduction, a system used in diesel engines to reduce emissions.
• Use an OBD-II scanner to confirm the code.
• Inspect the wiring and connectors for damage or corrosion.
• Test the reductant level sensor for proper operation.
• Replace the reductant level sensor if faulty.
• Repair or replace damaged wiring/connectors.
• Reprogram or replace the ECM if necessary.
• OBD-II scanner
• Multimeter
• Basic hand tools
$150 - $500
Moderate
• Ford F-250
• Chevrolet Silverado
• RAM 2500
• Regularly check and maintain DEF levels.
• Inspect wiring and connectors for corrosion.
• Ensure proper maintenance of the SCR system.
Sensor Issue
• Have the vehicle inspected by a professional if the Check Engine Light is on.
• Do not ignore the DEF warning messages as they can lead to reduced engine performance.
Check with your vehicle's manufacturer for warranty coverage on emissions-related components.
The P21AA code has been more common in diesel vehicles equipped with SCR systems due to stricter emissions regulations.
Moderate – Addressing this issue promptly can prevent further emissions system problems.
Ignoring emissions-related issues can result in failing emissions tests and potential fines.
• Vehicle must be running.
• DEF system must be active.
The reductant level sensor is typically located in or near the Diesel Exhaust Fluid (DEF) tank.
Q: Can I drive with a P21AA code?
A: While you may be able to drive, it's best to address the issue soon to avoid potential performance issues or emissions test failures.
Q: How do I reset the Check Engine Light after fixing the P21AA code?
A: Use an OBD-II scanner to clear the code after repairs are completed.
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