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 is having trouble getting a good reading from a sensor that checks how much special fluid, called reductant, is left in the tank. The sensor's signals are not steady or clear.
The P21AC code is triggered when the vehicle's engine control module (ECM) detects an intermittent or erratic signal from the reductant level sensor B circuit.
The reductant level sensor is part of the vehicle's Selective Catalytic Reduction (SCR) system, which reduces harmful emissions. Sensor B is responsible for monitoring the level of reductant (often diesel exhaust fluid) in the tank. An intermittent or erratic signal indicates that the sensor is not providing a consistent reading, which can affect the SCR system's performance.
• Check Engine Light is on
• Possible reduction in engine performance
• Increased emissions
• Faulty reductant level sensor
• Wiring or connector issues in the sensor circuit
• Malfunctioning ECM
Reductant: A fluid used in diesel engines to reduce emissions.
SCR: Selective Catalytic Reduction, a system that reduces harmful emissions from diesel engines.
• 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.
• Repair or replace damaged wiring or connectors.
• Replace the reductant level sensor if faulty.
• Update or reprogram the ECM if necessary.
• OBD-II scanner
• Multimeter
• Basic hand tools
$150 - $400
Moderate
• Ford F-250
• Chevrolet Silverado
• Ram 2500
• Regularly check and maintain reductant levels.
• Inspect sensor wiring for damage periodically.
• Follow manufacturer service schedules.
Sensor Issue
• Do not ignore the Check Engine Light.
• Consult a professional mechanic if unsure.
• Avoid driving long distances until fixed.
Check with the manufacturer for warranty coverage on emissions-related components.
This code has become more common with the increased use of SCR systems in diesel vehicles to meet emission standards.
Moderate – While the vehicle may still operate, emissions may be higher, and performance could be affected.
Driving with a malfunctioning emissions system may violate local emissions laws.
• Vehicle at operating temperature
• Stable electrical system voltage
The reductant level sensor is typically located in the reductant tank, which is part of the vehicle's SCR system.
Q: Can I drive with a P21AC code?
A: You may be able to drive, but it's best to fix the issue soon to avoid increased emissions and potential performance issues.
Q: How do I know if the reductant level sensor is faulty?
A: A professional mechanic can test the sensor using specialized tools to determine if it is working properly.
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