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 there is a problem with the electric switch that controls a part of the car's system that helps clean the exhaust. It's like a light switch that isn't working properly.
The P21C7 code indicates that the power relay circuit for the reductant control module is open or has a fault.
The reductant control module is responsible for managing the delivery of diesel exhaust fluid (DEF) into the exhaust system to reduce emissions. The power relay for this module is not functioning properly, which can prevent the system from operating as intended.
• Check Engine Light is on
• Reduced engine performance
• Increased emissions
• Faulty reductant control module power relay
• Open or short circuit in the relay control wiring
• Blown fuse for the reductant control module
Reductant: A substance used to reduce emissions in diesel engines, typically diesel exhaust fluid (DEF).
Relay: An electrical switch that opens and closes under the control of another electrical circuit.
• Check for any blown fuses related to the reductant system.
• Inspect the wiring and connectors for damage or corrosion.
• Test the power relay for proper operation.
• Replace the faulty power relay.
• Repair or replace damaged wiring or connectors.
• Replace any blown fuses.
• Multimeter
• Relay tester
• Basic hand tools
$100 - $300
Moderate
• Ford F-250
• Chevrolet Silverado
• Ram 2500
• Regularly inspect wiring and connectors for damage.
• Ensure fuses are in good condition.
• Perform routine maintenance on the reductant system.
Circuit/Open Issue
• Have the vehicle diagnosed by a professional.
• Avoid ignoring the Check Engine Light.
• Schedule regular maintenance checks.
Check with your vehicle manufacturer for warranty coverage on emissions-related components.
This code has been more common in diesel vehicles with selective catalytic reduction (SCR) systems.
Moderate – While the vehicle may still be drivable, emissions and performance can be affected.
Driving with a malfunctioning emissions system can lead to increased pollution and may not comply with emissions regulations.
• Vehicle at operating temperature
• Ignition on, engine off
The reductant control module and its relay are usually located near the DEF tank or in the engine compartment.
Q: Can I drive with a P21C7 code?
A: You can drive, but it's not recommended as it may lead to increased emissions and reduced performance.
Q: Will a P21C7 code affect fuel economy?
A: Yes, it can affect fuel economy due to inefficient emissions control.
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