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 left in the tank. This fluid helps reduce pollution from the car.
The P21AD code indicates an issue with the circuit of the reductant level sensor C, which monitors the level of the diesel exhaust fluid (DEF) used in diesel engines to reduce emissions.
The reductant level sensor C is part of the Selective Catalytic Reduction (SCR) system in diesel engines. It measures the amount of DEF available in the tank. If the sensor or its circuit malfunctions, it can lead to improper DEF levels being reported, affecting the SCR system's ability to reduce NOx emissions.
• Check Engine Light illuminated
• Reduced engine performance
• Increased emissions
• Faulty reductant level sensor
• Damaged wiring or connectors
• Defective SCR control module
Reductant: A substance that reduces emissions, in this case, Diesel Exhaust Fluid.
SCR: Selective Catalytic Reduction, a system used to reduce vehicle emissions.
• Use an OBD-II scanner to confirm the code.
• Inspect the wiring and connectors for damage.
• Test the reductant level sensor for proper operation.
• Replace the reductant level sensor if faulty.
• Repair or replace damaged wiring or connectors.
• Replace the SCR control module if necessary.
• OBD-II scanner
• Multimeter
• Basic hand tools
$150 - $500
Moderate
• Ford F-250
• Chevrolet Silverado 2500
• Ram 2500
• Regularly check and maintain DEF levels.
• Inspect wiring and connectors during routine maintenance.
• Use high-quality DEF fluid.
Sensor Issue
• Do not ignore the Check Engine Light.
• Have the vehicle inspected by a professional if unsure about the repair.
• Keep a spare bottle of DEF fluid in the vehicle.
Check with your vehicle manufacturer for warranty coverage on emissions-related components.
This code has become more common as emissions regulations have tightened and more vehicles use SCR systems.
Moderate – Addressing this issue promptly can prevent increased emissions and potential engine performance issues.
Operating a vehicle with malfunctioning emissions systems may violate emissions regulations and contribute to environmental pollution.
• Engine running
• Vehicle in a safe location
The reductant level sensor is typically located in or near the DEF tank, often at the rear of the vehicle.
Q: Can I drive with a P21AD code?
A: While the vehicle may still be drivable, it's best to address the issue promptly to avoid increased emissions and potential engine performance problems.
Q: How often should I check my DEF levels?
A: It's a good practice to check DEF levels at every oil change or as recommended by your vehicle's manufacturer.
High-End · Full-system diagnostics
A full-system bidirectional platform aimed at advanced DIY users and professional diagnostic workflows.
Network faults often require full-system access instead of a generic code reader alone.
View on Amazon
Universal · Digital multimeter
A solid entry-level multimeter for voltage, continuity, resistance, and basic electrical troubleshooting.
Helpful for checking voltage drops, grounds, and basic circuit health when modules are not communicating.
View on Amazon
Universal · Wire connectors and repair kit
Useful when diagnosis points toward damaged wiring, corroded terminals, or connector repairs.
Useful if diagnosis points toward damaged connectors, wiring, or terminal repairs.
View on AmazonThese are optional diagnostic tools, not guaranteed fixes. As an Amazon Associate I earn from qualifying purchases.