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 thinks there's a problem with a sensor that checks how much special fluid is in the tank. It's like the car can't tell if the tank is full or empty.
The P21AB code is triggered when the vehicle's Engine Control Module (ECM) detects a high voltage signal from the reductant level sensor B circuit, indicating a potential issue with the sensor or its circuit.
The reductant level sensor is part of the Diesel Exhaust Fluid (DEF) system, which helps reduce emissions in diesel engines. The sensor monitors the level of DEF in the tank. A 'circuit high' condition suggests that the sensor is sending a voltage signal that is higher than expected, which could be due to a sensor fault, wiring issue, or a problem with the ECM.
• Check Engine Light illuminated
• Possible DEF warning light
• Vehicle may enter limp mode
• Faulty reductant level sensor
• Wiring issues such as open or short circuits
• Faulty ECM (Engine Control Module)
Reductant: A substance used to reduce emissions in diesel engines, often Diesel Exhaust Fluid (DEF).
DEF: Diesel Exhaust Fluid, a liquid 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.
• Repair or replace damaged wiring or connectors.
• Replace the reductant level sensor if faulty.
• Reprogram or replace the ECM if necessary.
• OBD-II scanner
• Multimeter
• Basic hand tools
$100 - $500
Moderate
• Ford F-250
• Chevrolet Silverado
• Ram 2500
• Regularly check and maintain DEF levels.
• Inspect wiring and connectors periodically.
• Ensure proper DEF fluid quality is used.
Sensor Issue
• Do not ignore the Check Engine Light.
• Have the vehicle inspected by a professional if unsure.
• Avoid driving long distances until the issue is resolved.
Check vehicle warranty for coverage on emissions-related components.
This code has become more common with the increased use of DEF systems in modern diesel vehicles.
Moderate – Should be addressed soon to avoid potential drivability issues.
Driving with a malfunctioning emissions system may violate emissions regulations.
• Engine running
• DEF system active
The reductant level sensor is typically located in the DEF tank, often at the rear of the vehicle.
Q: Can I drive with a P21AB code?
A: It's not recommended to drive for long periods with this code, as it could lead to reduced engine performance.
Q: How do I reset the P21AB code?
A: After repairs, use an OBD-II scanner to clear the code. If the issue is fixed, it should not return.
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