Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
Your car has a special system to help clean the air by reducing bad stuff coming out of the exhaust. This code means there's a problem with the power that controls this system.
The P21C8 code indicates that there is a low voltage issue in the circuit that powers the reductant control module, which is responsible for managing the diesel exhaust fluid system.
The P21C8 code is set when the Engine Control Module (ECM) detects a low voltage condition in the control circuit of the reductant control module power relay. This module is part of the Selective Catalytic Reduction (SCR) system, which reduces nitrogen oxide emissions in diesel engines.
• Check Engine Light is on
• Reduced engine performance
• Possible increase in emissions
• Faulty reductant control module power relay
• Open or short circuit in the wiring
• Faulty ECM
ECM: Engine Control Module
SCR: Selective Catalytic Reduction
• Use an OBD-II scanner to verify the code.
• Inspect the wiring and connectors for damage or corrosion.
• Test the reductant control module power relay for proper operation.
• Replace the faulty reductant control module power relay.
• Repair or replace damaged wiring or connectors.
• 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 inspect and maintain the exhaust system.
• Ensure proper electrical connections and wiring integrity.
• Use high-quality diesel exhaust fluid.
Emission Control System Issue
• Do not ignore the check engine light.
• Get the vehicle inspected by a professional mechanic.
• Avoid driving long distances until the issue is resolved.
Check with the vehicle manufacturer for warranty coverage on emission-related components.
This code has been associated with issues in diesel vehicles equipped with SCR systems since their introduction.
Moderate – Addressing this issue promptly can prevent further emission system damage.
Driving with an active check engine light may result in failed emissions tests and increased pollution.
• Engine running
• Vehicle at operating temperature
The reductant control module is typically located near the exhaust system or under the vehicle.
Q: Can I drive my car with a P21C8 code?
A: It's not recommended, as it may lead to increased emissions and potential damage to the emission system.
Q: How can I reset the P21C8 code?
A: You can use an OBD-II scanner to clear the code, but it's important to fix the underlying issue first.
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.