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 found a problem with the power circuit that controls a part of the system that helps reduce pollution. It's like a switch that isn't working right.
The P21C9 code is triggered when the vehicle's engine control module (ECM) detects a high voltage condition in the power relay circuit of the reductant control module.
The reductant control module is part of the vehicle's emissions control system, specifically for diesel engines using Selective Catalytic Reduction (SCR) technology. The power relay is responsible for providing power to the module. A 'high' condition indicates that the voltage is above the expected range, which could be due to a short circuit or a faulty relay.
• Check Engine Light illuminated
• Possible reduction in engine performance
• Increased emissions or failure to meet emissions standards
• Faulty reductant control module power relay
• Short circuit in the wiring harness
• Faulty ECM or software issue
Reductant: A substance used in the SCR system to reduce emissions.
SCR: Selective Catalytic Reduction, a technology used to reduce nitrogen oxide emissions in diesel engines.
• Use an OBD-II scanner to confirm the code.
• 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.
• Update or reprogram the ECM if necessary.
• OBD-II scanner
• Multimeter
• Screwdriver set
$100 - $300
Moderate
• Ford F-Series
• Chevrolet Silverado
• Ram 2500
• Regularly inspect and maintain the vehicle's electrical system.
• Ensure all software updates are applied.
• Keep the engine bay clean to prevent corrosion.
Electrical Circuit Issue
• Do not ignore the Check Engine Light.
• Consult a professional mechanic if unsure about repairs.
Check with the vehicle manufacturer for warranty coverage on emissions-related components.
This code has been associated with vehicles using SCR systems since the introduction of stricter emissions standards.
Moderate – Address the issue to prevent increased emissions and potential engine performance issues.
Driving with a malfunctioning emissions system may violate emissions laws and regulations.
• Engine running
• Vehicle at operating temperature
The reductant control module is typically located near the exhaust system or in the engine bay.
Q: Can I drive with a P21C9 code?
A: It's possible to drive short distances, but it's best to fix the issue to avoid increased emissions and potential engine problems.
Q: Is a P21C9 code serious?
A: While not immediately critical, it should be addressed to maintain emissions compliance and engine performance.
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