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 a sensor in your car that checks the level of a special fluid used to clean the exhaust is not working properly. It's like a broken gauge that can't tell how much is left in the tank.
The reductant level sensor B is responsible for measuring the amount of diesel exhaust fluid (DEF) in the tank. If it gets stuck, it cannot provide accurate readings.
The reductant level sensor B is part of the Selective Catalytic Reduction (SCR) system that uses DEF to reduce emissions. When this sensor is stuck, the vehicle's computer cannot accurately determine the DEF level, potentially leading to emission control issues.
• Check Engine Light is on
• Incorrect DEF level readings
• Possible reduction in engine performance
• Faulty reductant level sensor
• Wiring issues or poor connections
• Contaminated DEF fluid
Reductant: A substance that reduces emissions in the exhaust system.
DEF: Diesel Exhaust Fluid, a solution used in SCR systems to reduce emissions.
• Check for any stored codes and freeze frame data.
• Inspect the wiring and connections to the reductant level sensor.
• Test the sensor for proper operation.
• Replace the faulty reductant level sensor.
• Repair or replace damaged wiring or connectors.
• Flush and replace contaminated DEF fluid.
• OBD-II scanner
• Multimeter
• Basic hand tools
$150 - $400
Moderate
• Ford F-Series
• Chevrolet Silverado
• Ram 1500
• Regularly check DEF fluid levels.
• Use high-quality DEF fluid.
• Inspect sensor connections periodically.
Sensor Issue
• Do not ignore the Check Engine Light.
• Have the vehicle inspected by a professional.
• Maintain regular vehicle service intervals.
Check with your vehicle 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 – It should be addressed soon to avoid potential emission test failures.
Ignoring this issue may lead to increased emissions, which can be illegal in some regions.
• Vehicle must be running.
• DEF tank should have fluid.
The reductant level sensor is typically located in the DEF tank, often towards the bottom or side.
Q: Can I drive with a P21C6 code?
A: While the car may still be drivable, it's best to address this issue soon to avoid potential emission problems.
Q: What happens if I ignore this code?
A: Ignoring the code can lead to inaccurate DEF levels, potentially causing the vehicle to limit performance to reduce emissions.
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A wired color-screen reader with readiness hotkeys, live data, code definitions, and data logging.
Good for emissions-related codes because it exposes readiness monitors and live powertrain data.
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Specialty · Smoke machine
Helpful for finding EVAP and vacuum leaks that are hard to see with visual inspection alone.
Helpful for tracking down EVAP and vacuum leaks that can trigger emissions faults.
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Specialty · O2 sensor socket
Helpful when the code points toward exhaust or oxygen-sensor work and space is tight.
Useful when the diagnosis points toward oxygen-sensor removal or replacement.
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