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 reading how much special fluid is left in the tank that helps clean the car's exhaust.
The P21A8 code is triggered when the Engine Control Module (ECM) detects a malfunction in the circuit of the reductant level sensor B. This sensor is responsible for measuring the level of Diesel Exhaust Fluid (DEF) in the tank.
The reductant level sensor B circuit monitors the amount of Diesel Exhaust Fluid (DEF) in the tank. This fluid is used in vehicles with Selective Catalytic Reduction (SCR) systems to reduce nitrogen oxide emissions. A fault in this circuit can lead to improper DEF levels being reported to the ECM, potentially affecting emissions control.
• Check Engine Light is on
• Warning message for DEF level
• Possible reduction in engine performance
• Increased emissions
• Faulty reductant level sensor B
• Wiring or connector issues in the sensor circuit
• DEF tank is empty or low
• ECM software issue
Reductant: A substance used to reduce emissions in diesel engines, often Diesel Exhaust Fluid (DEF).
SCR: Selective Catalytic Reduction, a system that reduces nitrogen oxide emissions in diesel engines.
• Check the DEF level in the tank
• Inspect the wiring and connectors for damage or corrosion
• Test the reductant level sensor B for proper operation
• Check for software updates for the ECM
• Refill the DEF tank if low or empty
• Repair or replace damaged wiring or connectors
• Replace the reductant level sensor B if faulty
• Update the ECM software if needed
• OBD-II scanner
• Multimeter
• Basic hand tools
$100 - $400
Moderate
• Ford F-250
• Chevrolet Silverado 2500
• Dodge Ram 2500
• Regularly check and refill DEF
• Inspect wiring and connectors periodically
• Follow maintenance schedule for SCR system
Sensor Issue
• Do not ignore the DEF warning light
• Schedule regular maintenance for the SCR system
Check vehicle warranty for coverage on emissions-related components.
This code has become more common with the increasing use of SCR systems in diesel vehicles to meet emissions standards.
Moderate – Addressing this issue promptly can prevent further emissions problems and potential engine performance issues.
Operating a vehicle with malfunctioning emissions control systems may violate emissions regulations.
• Vehicle running
• DEF tank not empty
The reductant level sensor is typically located in or on the DEF tank.
Q: Can I drive with a P21A8 code?
A: It's possible to drive, but it's recommended to fix the issue soon to avoid increased emissions and potential engine performance issues.
Q: How often should I refill the DEF tank?
A: The DEF tank should be refilled as needed, typically during regular maintenance intervals or when the warning light indicates low fluid.
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