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 part of the car that helps reduce pollution is getting too much electricity.
The P21CC code indicates that the voltage supplied to the reductant control module is higher than expected.
The reductant control module is responsible for managing the delivery of diesel exhaust fluid (DEF) to the exhaust system to reduce emissions. A high supply voltage can indicate a problem with the power supply or wiring, which can affect the module's operation.
• Check engine light is on
• Possible reduction in engine performance
• Increased emissions
• Faulty alternator causing overcharging
• Wiring issues such as short circuits
• Defective reductant control module
Reductant: A substance used to reduce emissions in diesel engines, often diesel exhaust fluid.
Voltage: The electrical potential difference, which powers electronic components.
• Check the battery and alternator voltage output.
• Inspect wiring and connectors for damage or corrosion.
• Test the reductant control module for proper operation.
• Replace or repair damaged wiring.
• Replace the reductant control module if faulty.
• Ensure the alternator is functioning correctly.
• Multimeter
• OBD-II scanner
• Wiring repair tools
$150 - $500
Moderate
• Ford F-250
• Chevrolet Silverado
• Ram 2500
• Regularly check and maintain battery and alternator health.
• Inspect wiring for wear and tear.
• Ensure proper maintenance of the reductant system.
Electrical Issue
• Avoid driving for long periods with the check engine light on.
• Schedule a diagnostic check with a professional.
• Monitor vehicle performance for any changes.
Check if the vehicle's emissions system is under warranty for possible coverage.
This code has been associated with issues in vehicles with advanced emission control systems, particularly diesel engines.
Moderate – Address the issue soon to prevent potential damage to the emissions system.
Ignoring emissions-related issues can lead to failing emissions tests and potential fines.
• Engine running at normal operating temperature
• Use of an OBD-II scanner to monitor voltage levels
The reductant control module is typically located near the DEF tank or within the engine compartment.
Q: What happens if I ignore this code?
A: Ignoring this code can lead to increased emissions and potential engine performance issues.
Q: Can I drive my car with this code?
A: While the car may still be drivable, it's best to address the issue to avoid further damage.
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.